Mum figures it's a motto (5)
I believe the answer is:
maxim
'a motto' is the definition.
(thesaurus)
'mum figures' is the wordplay.
I cannot quite understand how this works, but
'mum' could be 'ma' (both can mean mother) and 'ma' is located in the answer.
This explanation may well be incorrect...
'it's' is the link.
Can you help me to learn more?
(Other definitions for maxim that I've seen before include "Short pithy saying" , "Saying - inventor of machine gun" , "Apophthegm" , "Widely accepted saying" , "Pithy statement of a general truth" .)