Car safety feature (4,4)
(Other definitions for seatbelt that I've seen before include "compulsory 30 years ago" , "Safety feature" .)
'car safety feature' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for seat belt that I've seen before include "Safety strap in car - must wear it" , "Safety device in car - wearing mandatory" , "Safety device worn in car" , "Safety strap worn in car" , "became mandatory ..." .)