Bird got by sailor -- mighty, not half! -- in vessel (9)
I believe the answer is:
Here is my best explanation:
'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'.
'tarmig' put within 'pan' is 'PTARMIGAN'.
'got by' is the link.
(Other definitions for ptarmigan that I've seen before include "Northern grouse", "Game bird living the high life", "Grouse - giant pram (anag)", "Arctic grouse", "Type of mountain grouse", "Game bird, white in winter", "Mountain grouse", "Northern game bird - map rating (anag)", "Highland game bird -- giant ramp (anag)" and "One flying".)