Builder, mother with child (5)
I believe the answer is:
mason
'builder' is the definition.
(thesaurus)
'mother with child' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'with' means one lot of letters go next to another.
'child' becomes 'son' (son is a kind of child**).
'ma'+'son'='MASON'
(Other definitions for mason that I've seen before include "Stone-worker" , "one in construction" , "Construction industry employee" , "One in lodge" , "secretive lodger!" .)