Mum figures it's a boy (5)
I believe the answer is:
maxim
'a boy' is the definition.
'mum figures' is the wordplay.
I cannot really understand how this works, but
'mum' could be 'ma' (ma can mean mother) and 'ma' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for maxim that I've seen before include "Gorky saw" , "Widely accepted saying" , "- Gorky, Russian writer" , "Saw - type of gun" , "Old machine-gun" .)