Onesie fasteners (5)
I believe the answer is:
snaps
'onesie fasteners' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for snaps that I've seen before include "Informal photographs" , "Breaks in two" , "Breaks suddenly" , "Breaks under tension" , "Spells of cold weather" .)