Mariner, seafarer (6)
I believe the answer is:
sailor
I believe this is a double definition.
'mariner' is the first definition.
(I've seen this before)
'seafarer' is the second definition.
(I've seen this before)
(Other definitions for sailor that I've seen before include "Jack" , "Tar, AB" , "Matelot" , "Sinbad, for example" , "One in the main" .)