A grim main course? (6)
I believe the answer is:
astern
'main course?' is the definition.
Both the definition and answer are adverbs. Perhaps you can see a link between them that I can't see?
'a grim' is the wordplay.
'grim' becomes 'stern' (synonyms).
'a'+'stern'='ASTERN'
Can you help me to learn more?
(Other definitions for astern that I've seen before include "Behind at sea" , "Aft" , "The wake is" , "Toward the tail of a vessel" , "Behind the back of a ship" .)