A salt? Two salts! (6)
I believe the answer is:
tartar
'a salt?' is the definition.
(tartar is a kind of salt)
'two salts' is the wordplay.
'two salts' can mean two substitutions for 'salt'.
'salt' becomes 'tar' (both can informally mean a sailor).
'salt' becomes 'tar' (both can informally mean a sailor).
'tar'+'tar'='TARTAR'
(Other definitions for tartar that I've seen before include "Unmanageable sort" , "Crusty deposits on teeth" , "may be hard to brush off" , "Dental deposit" , "Hard, crusty deposit on the teeth" .)