Mother figures it's an old saw (5)
I believe the answer is:
'old saw' is the definition.
(I've seen this in another clue)
'mother figures it's an' is the wordplay.
I cannot really understand how this works, but
'mother' could be 'ma' (Ma is another word for mother or mum) and 'ma' is found within the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for maxim that I've seen before include "Gorky saw" , "Old machine-gun" , "- Gorky, Russian writer" , "Proverb, aphorism" , "Apophthegm" .)