Part of a gun (7)
I believe the answer is:
trigger
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">part of a gun</span>' is the definition.<br/ ><span class="explanation-format__explanation">('trigger' is part of 'gun')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for trigger that I've seen before include "Equine film star" , "Activating device" , "Part of gun" , "Lever to activate firing mechanism" , "Component of gun" .)