Onesie fastener (4)
I believe the answer is:
snap
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">onesie fastener</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)