But for the timely <em><u>arrival/ intervention</u></em> of the law enforcement agencies, there would have been a clash between the two communities.
Explanation:
In the given statement, the idea is that there was a possible clash between the two communities. But this confrontation was evidently thwarted by the enforcement agencies who managed to prevent the scene from unfolding into a huge brawl.
The use of the word <em>"but for the ....."</em> is meant to suggest "if not for ..." It is another way or form of saying "had not the enforcement agencies arrived on time".
Thus, the full statement will be <em>"but for the timely </em><u><em>arrival/ intervention</em></u><em> of the law enforcement agencies, there would have been a clash between the two communities".</em>