Item worn by biker slipped round youth (4,3)
I believe the answer is:
skid lid
'item worn by biker' is the definition.
(type of helmet)
'slipped round youth' is the wordplay.
'slipped' becomes 'slid' ('slide' can be a synonym of 'slip').
'round' is an insertion indicator.
'youth' becomes 'kid'.
'slid' enclosing 'kid' is 'SKID LID'.
(Other definitions for skid lid that I've seen before include "Protective cover" , "safety gear" , "Crash helmet (slang)" .)