Theirs is a growing business (7)
I believe the answer is:
farmers
'theirs is a growing' is the definition.
I can't judge whether this definition defines the answer.
'business' is the wordplay.
I cannot really see how this works, but
'business' could be 'farm' (I have seen 'agricultural business' mean 'farm' so perhaps 'business' could also mean 'farm') and 'farm' is found within the answer.
The remaining letters 'ers' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for farmers that I've seen before include "Agronomists" , "Cultivators of land" .)