Standard cart reversed by Rod (9)
I believe the answer is:
yardstick
'standard' is the definition.
(yardstick is a kind of standard)
'cart reversed by rod' is the wordplay.
'cart' becomes 'dray' (a cart without sides for delivering heavy loads).
'reversed' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'rod' becomes 'stick' (rod is a type of stick).
'dray' back-to-front is 'yard'.
'yard'+'stick'='YARDSTICK'
(Other definitions for yardstick that I've seen before include "Benchmark" , "Criterion - means of measuring" , "General measure for comparison" , "measure of performance" , "Three-foot rod as standard of comparison" .)