Herb and Tom, they make money here (7)
I believe the answer is:
catmint
'they make money here' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'herb and tom' is the wordplay.
'herb' becomes 'mint' (mint is a kind of herb).
'and' is a charade indicator (letters next to each other) (I've seen this in other clues).
'tom' becomes 'cat' (I have seen 'A tom ' mean 'cat' so perhaps 'tom' could also mean 'cat').
'mint' put after 'cat' is 'CATMINT'.
Can you help me to learn more?
(Other definitions for catmint that I've seen before include "Herb attractive to pets" , "Aromatic herb - Tim can't (anag)" , "Pungent garden plant" , "Plant appealing to felines" , "Plant liked by moggies" .)