Mum's working for a member of a secret society (5)
I believe the answer is:
mason
'a member of a secret society' is the definition.
(member of the Freemasons)
'mum's working' is the wordplay.
'mum's' becomes 'ma's' ('ma' can be a synonym of 'mum').
'working' becomes 'on'.
'mas'+'on'='MASON'
'for' acts as a link.
(Other definitions for mason that I've seen before include "US comedian" , "Secret society member" , "Construction industry employee" , "Actor James" , "secretive lodger!" .)