Bent cop's car carries heroin, a reliable money-maker (4,4)
I believe the answer is:
cash crop
'a reliable money-maker' is the definition.
The definition suggests a singular noun which matches the answer.
'bent cop's car carries heroin' is the wordplay.
'bent' indicates an anagram.
'carries' indicates putting letters inside (inserted letters carried inside).
'heroin' becomes 'H' (H is a slang term for heroin).
'cops'+'car'='copscar'
'copscar' with letters rearranged gives 'cascrop'.
'cascrop' enclosing 'h' is 'CASH CROP'.
Can you help me to learn more?
(Other definitions for cash crop that I've seen before include "Harvest for sale rather than consumption" , "Commercial farm product" , "Profitable farm produce" , "Ready-saleable harvest" , "What the grower will sell" .)