Grim aunt possibly becoming mellow (8)
I believe the answer is:
'becoming mellow' is the definition.
I can't judge whether this definition defines the answer.
'grim aunt possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'grimaunt' is an anagram of 'MATURING'.
Can you help me to learn more?
(Other definitions for maturing that I've seen before include "Growing, ripening", "Growing ripe or developing", "Growing ripe, reaching full growth".)