Onesie fastener (4)
I believe the answer is:
snap
'onesie fastener' 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 like simple card game" , "Collapse" , "A walk in the park < to shoot" , "Photograph - without warning" , "Go to pieces < , it's said, quickly" .)