Musk perhaps grim with nothing on (5)
I believe the answer is:
odour
'musk perhaps' is the definition.
(musk is a kind of odour)
'grim with nothing on' is the wordplay.
'grim' becomes 'dour' (synonyms).
'with' is a charade indicator (letters next to each other).
'nothing on' becomes 'o' (I've seen this in other clues).
'dour' put after 'o' is 'ODOUR'.
(Other definitions for odour that I've seen before include "Perfume" , "Regard" , "Aroma or fragrance" , "reputation" , "Smell, redolence" .)