From the game's encyclopedia
It's not uncommon for Rulers involved in a Struggle to get into border disputes and petty quarrels, changing with the nature of the current Struggle Phase. The ready availability of niche local disputes allows Rulers to conduct small-scale land grabs, conquering or vassalizing their targets.
Since Clashes depend on the environment of their Struggle, their Prestige cost scales with the current Struggle Phase.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Struggle Clash is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "It's not uncommon for Rulers involved in a Struggle to get into border disputes and petty quarrels, changing with the nature of the current Struggle Phase." It is part of Struggle. The game also links it as Clash and Clashes. Patch 1.20.1.2, from game/common/game_concepts/03_fp2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/03_fp2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.