Distance covered reversing, say, a cart (7)
I believe the answer is:
yardage
'distance covered' is the definition.
(I've seen this before)
'reversing say a cart' is the wordplay.
'reversing' says the letters should be written backwards.
'say' becomes 'eg'.
'cart' becomes 'dray' (a cart without sides for delivering heavy loads).
'eg'+'a'+'dray'='egadray'
'egadray' backwards is 'YARDAGE'.
(Other definitions for yardage that I've seen before include "Distance - day rage (anag)" , "charge for using court" , "Distance measured in imperial units" , "Distance gained (Am. football)" , "Distance covered" .)