Boy with unknown noble woman (4)
I believe the answer is:
lady
'noble woman' is the definition.
'boy with unknown' is the wordplay.
'boy' becomes 'lad' (lad is a kind of boy**).
'with' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
'lad'+'y'='LADY'
(Other definitions for lady that I've seen before include "Wife of gentleman" , "Gentlewoman" , "See 7" , "Noble" , "mature woman" .)