Someone with a child (6)
I believe the answer is:
person
'someone' is the definition.
('person' can be a synonym of 'someone')
'with a child' is the wordplay.
'with' means one lot of letters go next to another.
'a' becomes 'per'.
'child' becomes 'son' (son is a kind of child).
'per'+'son' is 'PERSON'.
(Other definitions for person that I've seen before include "he's the third" , "A man or woman" , "Type" , "present appearing in this" , "character" .)