Demanding firm (4)
I believe the answer is:
hard
This clue is a double definition.
'demanding' is the first definition.
('hard' can be similar in meaning to 'demanding')
'firm' is the second definition.
('hard' can be similar in meaning to 'firm')
(Other definitions for hard that I've seen before include "Difficult; rigid" , "Tough - difficult" , "Firm, rigid, road leading to foreshore" , "to be listened to in Scotland" , "Newspaper's sure" .)