Indisposed at sea, when heading out (6)

I believe the answer is:

ailing

'indisposed' is the definition.
('ailing' can be a synonym of 'indisposed')

I can't explain the rest of the clue.

(Other definitions for ailing that I've seen before include "In poor health" , "Infertile" , "feeling sick" , "not doing very well" , "Troubled in mind or body" .)

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app