Returning, share carriage (4)
I believe the answer is:
trap
'carriage' is the definition.
(I know that trap is a type of carriage)
'returning share' is the wordplay.
'returning' says the letters should be written backwards.
'share' becomes 'part' (synonyms).
'part' back-to-front is 'TRAP'.
(Other definitions for trap that I've seen before include "Vehicle" , "Snare - mouth" , "Ambush" , "Kisser" , "Trick" .)