Horribly grim aunt mellowing perhaps (8)
I believe the answer is:
maturing
'mellowing perhaps' is the definition.
'horribly grim aunt' is the wordplay.
'horribly' is an anagram indicator.
'grim'+'aunt'='grimaunt'
'grimaunt' anagrammed gives 'MATURING'.
(Other definitions for maturing that I've seen before include "Growing, ripening" , "Growing ripe or developing" , "Growing ripe, reaching full growth" .)