Boy with unknown titled woman (4)
I believe the answer is:
lady
'titled woman' is the definition.
'boy with unknown' is the wordplay.
'boy' becomes 'lad' (lad is a kind of 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 "See 20" , "Noble" , "Lord's wife - peeress's title" , "Wife of gentleman" , "Female peer" .)