Close noisily (4)
I believe the answer is:
'close noisily' 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 "Close noisily" , "A grand one is 13 tricks" , "If it's grand, it wins all 13 tricks in bridge game" , "Shut hard" , "Crash (door closed)" .)