Plant firm after Cabot returns (7)
I believe the answer is:
tobacco
'plant' is the definition.
(tobacco is a kind of plant)
'firm after cabot returns' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'after' means one lot of letters go next to another.
'returns' says the letters should be written in reverse.
'cabot' reversed gives 'tobac'.
'co' after 'tobac' is 'TOBACCO'.
(Other definitions for tobacco that I've seen before include "Pipe leaves" , "(Treated leaves of) American plant" , "Leaf for smoking" , "Leaves prepared for smoking" , "make a cot cob from this plant" .)