Grim aunt possibly becoming mellow (8)
I believe the answer is:
maturing
'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.
'grim'+'aunt'='grimaunt'
'grimaunt' is an anagram of 'MATURING'.
Can you help me to learn more?
(Other definitions for maturing that I've seen before include "Growing ripe or developing" , "Growing ripe, reaching full growth" , "Growing, ripening" .)