Labour leader making a change, mostly wrong about everything (9)
I believe the answer is:
callaghan
'labour leader' is the definition.
The definition suggests a singular noun which matches the answer.
'a change mostly wrong about everything' is the wordplay.
'mostly' means to remove the last letter.
'wrong' is an anagram indicator.
'about' indicates putting letters inside.
'everything' becomes 'alla' (I can't explain this - if you can you should believe this answer much more).
'change' with its last letter taken off is 'chang'.
'chang' with letters rearranged gives 'cghan'.
'cghan' enclosing 'alla' is 'CALLAGHAN'.
'making' acts as a link.
Can you help me to learn more?
(Other definitions for callaghan that I've seen before include "3's Labour predecessor" , "Former PM" , "Former prime minister" .)