Keep a Lid On is an achievement/trophy in L.A. Noire. It can be earned by completing a brawl without losing your hat after completing the Patrol desk.

The achievement can be earned on any case or Street Crime involving a a brawl, but among earliest and among the easiest opportunity to obtain the achievement is the Traffic street crime Boxing Clever. Nat Spencer is among the easiest melee combatants to defeat, among the biggest problems is to avoid getting too close to him when climbing the ladder as he may kick you in the head.