Spoil stuff on the way back (3)
I believe the answer is:
Here is my best explanation:
'spoil' is the definition.
('mar' can be a synonym of 'spoil')
'stuff on the way back' is the subsidiary indication.
'stuff' becomes 'ram' (ramming is a kind of stuffing).
'on the way back' says the letters should be written in reverse.
'ram' backwards is 'mar'.
(Other definitions for mar that I've seen before include "Detract from", "A windy month", "'Impair, spoil (3)'", "Spoil stuff", "Spoil or deface", "Ram into and spoil", "Spanish main?", "Damage, spoil", "'Damage, impair (3)'" and "Spoil, damage".)