Note date with unknown noblewoman (4)
I believe the answer is:
lady
'noblewoman' is the definition.
(I know that noblewoman can be written as Lady)
'note date with unknown' is the wordplay.
'note' becomes 'la' (musical note as in do-re-mi).
'date' becomes 'd' (common abbreviation eg in DOB, date of birth).
'with' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'la'+'d'+'y'='LADY'
(Other definitions for lady that I've seen before include "Female peer" , "Gentlewoman" , "Lord's partner" , "Lord's wife - peeress's title" , "Woman, of refinement perhaps" .)