Marx on track for coffee substitute (7)
I believe the answer is:
chicory
'coffee substitute' is the definition.
(I've seen this before)
'marx on track' is the wordplay.
'marx' becomes 'chico' (chico marx was a marx brother).
'on' means one lot of letters go next to another.
'track' becomes 'ry'.
'chico'+'ry'='CHICORY'
'for' is the link.
(Other definitions for chicory that I've seen before include "Plant used instead of coffee" , "used in salads" , "Plant with edible salad leaves" , "Salad plant (with a root ground and used in coffee)" , "Vegetable used with coffee" .)