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'.