Grim firm (4)
I believe the answer is:
hard
This clue is a double definition.
'grim' is the first definition.
(thesaurus)
'firm' is the second definition.
('hard' can be similar in meaning to 'firm')
(Other definitions for hard that I've seen before include "impenetrable" , "Difficult; severe" , "Newspaper's sure" , "Solid; tough" , "Emergency lane" .)