Much ardour inside firm (4)
I believe the answer is:
hard
'firm' is the definition.
('hard' can be similar in meaning to 'firm')
'much ardour inside' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'HARD' is hidden within 'much ardour'.
(Other definitions for hard that I've seen before include "Difficult - firm" , "Solid; difficult" , "Complicated" , "impenetrable" , "badly" .)