Bird got by sailor -- mighty, not half! -- in vessel (9)
I believe the answer is:
ptarmigan
'bird' is the definition.
(ptarmigan is a kind of bird)
'sailor mighty not half in vessel' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor**).
'not half' suggests halving the letters.
'in' means one lot of letters goes inside another.
'vessel' becomes 'pan' (I have seen 'Shallow vessel ' mean 'pan' so perhaps 'vessel' could also mean 'pan').
'mighty' cut in half is 'mig'.
'tar'+'mig'='tarmig'
'tarmig' put within 'pan' is 'PTARMIGAN'.
'got by' is the link.
(Other definitions for ptarmigan that I've seen before include "One flying" , "Game bird living the high life" , "Grouse - giant pram (anag)" , "Northern grouse" , "Game bird, white in winter" .)