Jill’s partner? He’s a sailor (4)
I believe the answer is:
jack
I believe this is a triple definition!
'jill's partner?' is the first definition.
(as in Jack and Jill, nursery rhyme characters)
'he's' is the second definition.
'a sailor' is the third definition.
(jack can informally mean a sailor)
(Other definitions for jack that I've seen before include "Bowls target; small flag" , "Lifting tool" , "elevator?" , "Target ball in bowls" , "Something for raising < flag on board" .)