Release call for peace by superpower (7)
I believe the answer is:
quietus
'release call' is the definition.
'quietus' can be an answer for 'release' (thesaurus). I'm not sure about the 'call' bit.
'for peace by superpower' is the wordplay.
'for peace' becomes 'quiet' (I can't justify this - if you can you should believe this answer much more).
'by' means one lot of letters go next to another.
'superpower' becomes 'US' (the United States is a global superpower).
'quiet'+'us'='QUIETUS'
Can you help me to learn more?
(Other definitions for quietus that I've seen before include "Extinction, death" , "Final riddance - not quite quite us" , "Curtains" , "passing away" , "discharge" .)