Mariner's part of the deck (4)
I believe the answer is:
jack
I believe this clue is a double definition.
'mariner's' is the first definition.
(thesaurus)
'the deck' is the second definition.
(thesaurus)
'part of' acts as a link.
(Other definitions for jack that I've seen before include "Jill's companion" , "bowler rolls it" , "Face card" , "elevator?" , "See 5 Down" .)