Mother's working as a builder (5)
I believe the answer is:
mason
'a builder' is the definition.
(I know that mason is a type of builder)
'mother's working as' is the wordplay.
'mother's' becomes 'ma's' (Ma is another word for mother or mum).
'working as' becomes 'on' (on can mean working or activated. I am not sure about the 'as' bit.).
'mas'+'on'='MASON'
Can you help me to learn more?
(Other definitions for mason that I've seen before include "Actor James" , "Craftsman working in stone" , "Stone carver" , "Jewish comedian" , "figure in lodge" .)