Sailor is a knave (4)
I believe the answer is:
jack
This clue is a double definition.
'sailor' is the first definition.
(informal term for a sailor)
'knave' is the second definition.
(I know that knave can be written as jack)
'is a' acts as a link.
(Other definitions for jack that I've seen before include "elevator?" , "Lifting tool" , "Knave (cards)" , "-- Sprat; -- Horner" , "Face card" .)