Seaman conceals grim smells (6)
I believe the answer is:
odours
'smells' is the definition.
('odour' can be a synonym of 'smell')
'seaman conceals grim' is the wordplay.
'seaman' becomes 'OS' (short for ordinary seaman).
'conceals' means one lot of letters goes inside another.
'grim' becomes 'dour' (synonyms).
'os' placed around 'dour' is 'ODOURS'.
(Other definitions for odours that I've seen before include "Aromas, pungencies" , "Smells and fragrances" , "bouquets" , "Smells, usually unpleasant" , "Smells, redolences" .)