Correspondence excluding girl and mother from legal matters (7)
I believe the answer is:
letters
'correspondence' is the definition.
(written correspondence)
'excluding girl and mother from legal matters' is the wordplay.
I cannot quite see how this works, but
'from' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
'matters' could be 'ers' (Er is an example) and 'ers' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for letters that I've seen before include "Characters" , "academic credentials" , "They are bound to look after tenants" , "...as, for example" , "Epistles" .)