Join a knight and a sailor (6)
I believe the answer is:
seaman
'sailor' is the definition.
(I know that seaman is a type of sailor)
'join a knight and a' is the wordplay.
'join' becomes 'seam' (seaming is a kind of joining).
'a knight' becomes 'N' (chess abbreviation).
'and' means one lot of letters go next to another.
'n' put after 'a' is 'an'.
'seam'+'an'='SEAMAN'
(Other definitions for seaman that I've seen before include "Sailor" , "A manse (anag.)" , "Mariner" , "crew member" .)