Restrain youngster crossing road initially (4)
I believe the answer is:
curb
'restrain' is the definition.
('curb' can be a synonym of 'restrain')
'youngster crossing road initially' is the wordplay.
'youngster' becomes 'cub' (I've seen this in other clues**).
'crossing' indicates putting letters inside.
'initially' suggests taking the first letters.
The initial letter of 'road' is 'r'.
'cub' going around 'r' is 'CURB'.
(Other definitions for curb that I've seen before include "Bit - restraint" , "'Temper, hold in restraint (4)'" , "Control, limit" , "Hold back or try to prevent" , "[LIMITATION]" .)