Daughter is not unknown to be genteel (6)
I believe the answer is:
dainty
'genteel' is the definition.
(thesaurus)
'daughter is not unknown' is the wordplay.
'daughter' becomes 'd'.
'is not' becomes 'aint' (I've seen this before).
'unknown' becomes 'y' (algebra).
'd'+'aint'+'y'='DAINTY'
'to be' acts as a link.
(Other definitions for dainty that I've seen before include "Choice" , "Small and pretty, delicate" , "Delicately small and pretty" , "Twee" , "Titbit, delicacy" .)