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 subsidiary indication.
I cannot quite understand how this works, but
'sailor' could be 'tar' (tar is a kind of sailor) and 'tar' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'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".)