Double dose of salt for old invader (6)
I believe the answer is:
tartar
'old invader' is the definition.
(someone from the historical region of Tartary)
'double dose of salt' is the wordplay.
'double dose of' says to double the letters.
'salt' becomes 'tar' (synonyms).
'tar' duplicated is 'TARTAR'.
'for' acts as a link.
(Other definitions for tartar that I've seen before include "Unpleasant sort" , "Fearsome person: kind of sauce" , "A ferocious person" , "plaque" , "Sauce often accompanying fish" .)