Prompt start (7)
I believe the answer is:
trigger
I believe this clue is a double definition.
'prompt' is the first definition.
(I've seen this before)
'start' is the second definition.
(I've seen this before)
(Other definitions for trigger that I've seen before include "Start" , "Roy Rogers' horse" , "arm with this?" , "prompt" , "Pull it to discharge gun" .)