Terribly late, socialist changed (7)
I believe the answer is:
altered
'changed' is the definition.
('alter' can be a synonym of 'change')
'terribly late socialist' is the wordplay.
'terribly' indicates an anagram.
'socialist' becomes 'red' (red can informally mean a socialist).
'late' is an anagram of 'alte'.
'alte'+'red'='ALTERED'
(Other definitions for altered that I've seen before include "Changed, adapted" , "Change" , "Treadle (anag)" , "Different" , "Metamorphosed" .)