Join a new mariner (6)
I believe the answer is:
seaman
'mariner' is the definition.
(I know that mariner can be written as seaman)
'join a new' is the wordplay.
'join' becomes 'seam' (seaming is a kind of joining).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'seam'+'a'+'n'='SEAMAN'
(Other definitions for seaman that I've seen before include "A manse (anag.)" , "Mariner" , "Sailor" , "crew member" .)