Start of a football play (4)
I believe the answer is:
snap
'start of a football play' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for snap that I've seen before include "Break with a crack" , "Bite at or utter angry words" , "Simple card game - gotcha!" , "Informal photograph" , "Break suddenly and cleanly" .)