Plant to mostly support firm (7)
I believe the answer is:
tobacco
'plant' is the definition.
(tobacco is a kind of plant)
'to mostly support firm' is the wordplay.
I cannot quite see how this works, but
'to' is within the answer.
'firm' could be 'co' (a firm is a company) and 'co' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tobacco that I've seen before include "New World product" , "Plant with narcotic leaves" , "Nicotine-rich plant" , "plugs, perhaps" , "Plant associated with serious health risks" .)