Below decks, finally, sick at sea (7)
I believe the answer is:
sailing
'at sea' is the definition.
(synonyms)
'below decks finally sick' is the wordplay.
'below' means one lot of letters go next to another (in a down clue, some letters go below others).
'decks finally' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'sick' becomes 'ailing' (similar in meaning).
's' next to 'ailing' is 'SAILING'.
Can you help me to learn more?
(Other definitions for sailing that I've seen before include "Water sport" , "event" , "(Of ship) leaving port" , "Going by, eg, yacht" , "Marine sport" .)