It's not mentioned that hearty aunts get like granny (6)
I believe the answer is:
untold
'hearty aunts get like granny' is the definition.
I can't tell whether this defines the answer.
'it's not mentioned' is the wordplay.
'it's not' becomes 'un' (prefix meaning 'not').
'mentioned' becomes 'told' (I can't justify this - if you can you should believe this answer much more).
'un'+'told'='UNTOLD'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for untold that I've seen before include "Not narrated or measured" , "Too many to be counted; not narrated" , "Story yet to be read" , "like a kept secret?" , "Incalculable amount" .)