Sailor using his head in final act on board (8)
I believe the answer is:
shipmate
'sailor' is the definition.
(I have seen 'Fellow sailor' mean 'shipmate' so perhaps 'sailor' could also mean 'shipmate')
'using his head in final act on board' is the wordplay.
'using' indicates an anagram.
'head in' becomes 'p' (I can't explain this - if you can you should give a lot more credence to this answer).
'final act on board' becomes 'mate' (move which ends a game of chess).
'his' anagrammed gives 'shi'.
'shi'+'p'+'mate'='SHIPMATE'
(Other definitions for shipmate that I've seen before include "Fellow seaman" , "Friend on board" , "Fellow crewman" , "Sailor on board, a real pal" , "Fellow crew member" .)