Fellow crew member - he maps it at sea (8)
I believe the answer is:
shipmate
'fellow crew member' is the definition.
(I've seen this before)
'he maps it at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'he'+'maps'+'it'='hemapsit'
'hemapsit' anagrammed gives 'SHIPMATE'.
(Other definitions for shipmate that I've seen before include "Colleague on board" , "Fellow crewman" , "Friend on board" , "Sailor on board, a real pal" , "Fellow seaman" .)