Although context should be provided to answer this question with certainty, it seems apparent that this is not a phrasal verb, but simply the verb "clash", which can mean "collide", "conflict", or "argue vehemently" (the latter is most probably the one you came across), and the adverb "over", which can here be replaced by "on the subject of". In short: "clash over" = "argue about" Hope this helps and don't hesitate to ask if you need further explanations.

"Have a conflict about/argue about." For example: "The sisters clashed over a boy." = "The sisters were in conflict for an extended period of time because of a boy."