Lover's knot, say (4)
I believe the answer is:
beau
'lover's' is the definition.
(beau is a kind of lover)
'knot say' is the wordplay.
'knot' becomes 'bow' (bow is a kind of knot).
'say' indicates a 'sounds like' (homophone) clue.
'bow' sounds like 'BEAU'.
(Other definitions for beau that I've seen before include "Dandy - lover" , "An old-fashioned boy friend - like Brummel?" , "An archaic boy-friend like Brummel" , "Fop; lover" , "Male lover of a girl or young woman" .)