Condiment for a mariner (4)
I believe the answer is:
salt
I believe this is a double definition.
'condiment' is the first definition.
(I know that salt is a condiment)
'for a mariner' is the second definition.
('salt' can be a synonym of 'mariner')
(Other definitions for salt that I've seen before include "Brackish" , "Mariner; sodium chloride" , "Crystalline substance used to season food" , "Common substance" , "White crystalline substance" .)