Believed socialist mentioned outside (8)
I believe the answer is:
credited
'believed' is the definition.
(I've seen this before)
'socialist mentioned outside' is the wordplay.
'socialist' becomes 'red' (red can informally mean a socialist).
'mentioned' becomes 'cited' ('cite' can be a synonym of 'mention').
'outside' means one lot of letters goes inside another (some letters go outside others).
'red' put within 'cited' is 'CREDITED'.
(Other definitions for credited that I've seen before include "3 across (anag.)" , "Acknowledged as writer" , "Esteemed" , "Believed" , "given improved balance" .)