Go separate ways when returning in carriage (4)
I believe the answer is:
trap
'carriage' is the definition.
(I know that Trap is a type of carriage)
'go separate ways when returning' is the wordplay.
'go separate ways' becomes 'part' (to part from someone is to leave them).
'when returning' shows that the letters should be reversed in order.
'part' written backwards gives 'TRAP'.
'in' acts as a link.
(Other definitions for trap that I've seen before include "Means of transport - sometimes sprung!" , "Light carriage; ensnare" , "Confine" , "Snare, ruse" , "Ambush" .)