Unwell below decks, finally at sea (7)
I believe the answer is:
sailing
'at sea' is the definition.
('sailing' can be a synonym of 'at sea')
'unwell below decks finally' is the wordplay.
'unwell' becomes 'ailing' ('ailing' can be a synonym of 'unwell').
'below' says to put letters next to each other.
'finally' says to take the final letters.
The last letter of 'decks' is 's'.
'ailing' put after 's' is 'SAILING'.
(Other definitions for sailing that I've seen before include "Eg, yachting" , "Water sport" , "event" , "Going by, eg, yacht" , "(Of ship) leaving port" .)