Unwell below decks, finally at sea (7)
I believe the answer is:
Here is my best explanation:
'at sea' is the definition.
('sailing' can be a synonym of 'at sea')
'unwell below decks finally' is the subsidiary indication.
'unwell' becomes 'ailing' ('ailing' can be a synonym of 'unwell').
'below' is a charade indicator (letters next to each other).
'finally' suggests 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 "Going by, eg, yacht", "Water sport", "Leaving port", "(Of ship) leaving port" and "Eg, yachting".)