Ulster, maybe remaining firm against extremists (8)
I believe the answer is:
overcoat
'ulster' is the definition.
(thesaurus)
'remaining firm against extremists' is the wordplay.
'remaining' becomes 'over' (as in 'we ate them all but had one over').
'firm' becomes 'co' (a firm is a company).
'extremists' says to hollow out the word (remove centre letters) (letters on the extreme ends of the word).
'against' with its middle removed is 'at'.
'over'+'co'+'at'='OVERCOAT'
'maybe' is the link.
(Other definitions for overcoat that I've seen before include "Protective winter garment" , "Ulster, perhaps" , "warmer" , "Warm garment" , "Heavy outer garment" .)