Child skated round in helmet (4,3)
I believe the answer is:
skid lid
'helmet' is the definition.
(I know that skid lid is a type of crash helmet)
'child skated round' is the wordplay.
'child' becomes 'kid' (synonyms).
'skated' becomes ''slid' (I've seen this in another clue).
'round' is an insertion indicator.
'kid' put within 'slid' is 'SKID LID'.
'in' is the link.
(Other definitions for skid lid that I've seen before include "safety gear" , "Crash helmet (slang)" , "Protective cover" .)