The unmarried one (6)
I believe the answer is:
single
I believe this clue is a double definition.
'the unmarried' is the first definition.
(thesaurus)
'one' is the second definition.
('single' can be a synonym of 'one')
(Other definitions for single that I've seen before include "Like bachelor" , "A hit for one run in cricket" , "Unattached" , "Free" , "Solo" .)