Label that may be applied to child's play (3)
I believe the answer is:
tag
This clue is a double definition.
'label that may be applied' is the first definition.
'child's play' is the second definition.
(I've seen this in another clue)
'to' acts as a link.
(Other definitions for tag that I've seen before include "stock quotation" , "Label - children's game" , "Identity disc" , "day in Berlin" , "Label (to attach)" .)