Cart around jam that's a good standard (9)
I believe the answer is:
yardstick
'a good standard' is the definition.
(a yardstick is a standard or guide)
'cart around jam' is the wordplay.
'cart' becomes 'dray' (a cart without sides for delivering heavy loads).
'around' shows that the letters should be reversed in order.
'jam' becomes 'stick' (jam can mean to get stuck).
'dray' back-to-front is 'yard'.
'yard'+'stick'='YARDSTICK'
'that's' is the link.
(Other definitions for yardstick that I've seen before include "Standard base for comparison" , "Three-foot rod as standard of comparison" , "Measuring rod" , "Benchmark" , "General measure for comparison" .)