Leaves firm after Cabot returns (7)
I believe the answer is:
tobacco
'leaves' is the definition.
(I've seen this before)
'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' back-to-front is 'tobac'.
'co' after 'tobac' is 'TOBACCO'.
(Other definitions for tobacco that I've seen before include "Eg, shag or snuff" , "Smoker's leaves" , "New World product" , "Smokers' 'weed'" , "Dangerous plant" .)