Boy with unknown noblewoman (4)
I believe the answer is:
lady
'noblewoman' is the definition.
(I know that noblewoman can be written as Lady)
'boy with unknown' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'with' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'lad'+'y'='LADY'
(Other definitions for lady that I've seen before include "Gentlewoman" , "mature woman" , "Lord's partner" , "Title for a knight's wife" , "Female peer" .)