Mother's working for fraternal society member (5)
I believe the answer is:
mason
'fraternal society member' is the definition.
(the Freemasons are a fraternal society)
'mother's working' is the wordplay.
I cannot quite understand how this works, but
'mother's' could be 'mas' (Ma is another word for mother or mum) and 'mas' is located in the answer.
'working' could be 'on' (on can mean working or activated) and 'on' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
(Other definitions for mason that I've seen before include "A worker in stone - may be free" , "One in lodge" , "One is skilful with stone" , "Craftsperson who works with stone or brick" , "figure in lodge" .)