Distance, say, a cart reversed (7)
I believe the answer is:
yardage
'distance' is the definition.
(distance measured in yards)
'say a cart reversed' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'cart' becomes 'dray' (a cart without sides for delivering heavy loads).
'reversed' is a reversal indicator.
'eg'+'a'+'dray'='egadray'
'egadray' in reverse letter order is 'YARDAGE'.
(Other definitions for yardage that I've seen before include "Length in old units" , "cost of storing" , "Distance gained (Am. football)" , "Length in units of three feet" , "particular distance" .)