What do you get for winning a Struggle Clash war in CK3?

… gains the contested Title (patch 1.20.0.3).

What does the Struggle Clash casus belli cost?

piety and prestige; the amount depends on the titles, claims and distance, worked out when you declare.

Casus belli · Struggles

Struggle Clash

Struggle Clash is a casus belli in Crusader Kings III (struggles). If the attacker wins: … gains the contested Title. If the defender wins: … keeps the contested Title. Declaring it costs piety and prestige. The war goal is worth 80% war score. Patch 1.20.0.3, from game/common/casus_belli_types/00_struggle_war.txt.

Quick answers (3)

What do you get for winning a Struggle Clash war in CK3?: … gains the contested Title (patch 1.20.0.3).
What happens on a white peace?: … keeps the contested Title.
What does the Struggle Clash casus belli cost?: piety and prestige; the amount depends on the titles, claims and distance, worked out when you declare.

How the war ends

Attacker wins

… gains the contested Title.

White peace

… keeps the contested Title.

Defender wins

… keeps the contested Title.

If the war becomes invalid

The … has ended: the defender no longer … the County.

Who can declare it

The game shows no text of its own for these conditions; the script is below.

Every struggles war (the group's allowed_for_character) (8 lines of game script)

can_use_struggle_cbs_trigger = yes
herders_and_tributary_constraints = yes
pam_personal_pacifism_offensive_war_ban_trigger = yes
tgp_dynastic_cycle_offensive_wars_ban_trigger = yes
tgp_war_ban_all_ministers_trigger = yes
NOT = {
	government_has_flag = government_is_theocracy
}

As written in game/common/casus_belli_groups/00_casus_belli_groups.txt.

Attacker (allowed_for_character) (3 lines of game script)

NOT = {
	government_has_flag = government_is_landless_adventurer
}

As written in game/common/casus_belli_types/00_struggle_war.txt.

Defender (allowed_against_character) (28 lines of game script)

OR = {
	AND = {
		exists = scope:attacker.house
		scope:attacker.house = {
			OR = {
				has_house_modifier = fp2_struggle_hostility_house_combined_modifier
				has_house_modifier = fp2_struggle_hostility_house_faith_modifier
				has_house_modifier = fp2_struggle_hostility_house_culture_modifier
			}
		}
	}
	scope:attacker = {
		any_character_struggle = {
			involvement = involved
			is_struggle_type = iberian_struggle
			is_secondary_character_involvement_either_trigger = {
				CHAR = scope:defender
			}
		}
	}
}
NOT = {
	scope:attacker = {
		any_liege_or_above = {
			this = scope:defender
		}
	}
}

As written in game/common/casus_belli_types/00_struggle_war.txt.

When it can start (valid_to_start) (6 lines of game script)

scope:target = {
	is_landless_type_title = no
	valid_struggle_cb_target_trigger = {
		CHARACTER = scope:attacker
	}
}

As written in game/common/casus_belli_types/00_struggle_war.txt.

Cost to declare

Costs piety and prestige. The amount is worked out when you declare (by the titles targeted, claims, distance and more), so the script is shown as written:

Cost (20 lines of game script)

piety = {
	value = 0
	if = {
		limit = {
			scope:defender.faith = scope:attacker.faith
		}
		value = struggle_cb_phase_modifier_cost
	}
	multiply = common_cb_piety_cost_multiplier_sans_struggles
}
prestige = {
	value = 0
	if = {
		limit = {
			scope:defender.faith != scope:attacker.faith
		}
		value = struggle_cb_phase_modifier_cost
	}
	multiply = common_cb_prestige_cost_multiplier_sans_struggles
}

As written in game/common/casus_belli_types/00_struggle_war.txt.

Source: game/common/casus_belli_types/00_struggle_war.txt and the English localisation, Crusader Kings III 1.20.0.3. “The attacker”, “the defender” stand where the game names the character; “…” marks other words it fills in during play.