Community workers side with morphing, vengeful trio (9)
I believe the answer is:
eumenides
'vengeful trio' is the definition.
(spirits of vengeance in Greek mythology)
'community workers side with morphing' is the wordplay.
'community' becomes 'eu' (European Union, a community of nations).
'workers' becomes 'men' (man is a kind of worker).
'with morphing' is an anagram indicator.
'side' is an anagram of 'ides'.
'eu'+'men'+'ides'='EUMENIDES'
(Other definitions for eumenides that I've seen before include "Women who are furious" , "Females furious" , "The Furies" , "Vengeful females" .)