Big bang? (4)
I believe the answer is:
slam
'big bang?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slam that I've seen before include "Crash (door closed)" , "A grand one is 13 tricks" , "Criticise - attack" , "Ambitious contract" , "Shut (a door) noisily" .)