Unwell below decks, finally at sea (7)

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'.