Joke from proletariat about Labour's leader (10)
I believe the answer is:
pleasantry
'joke' is the definition.
(I know that pleasantry is a type of joke)
'proletariat about labour's leader' is the wordplay.
'proletariat' becomes 'peasantry' (proletariat means the worker class).
'about' is an insertion indicator.
'labour's leader' becomes 'l'.
'peasantry' enclosing 'l' is 'PLEASANTRY'.
'from' acts as a link.
(Other definitions for pleasantry that I've seen before include "Sally" , "Slightly amusing remark" , "Quip" , "Humorous style" , "Jocular comment" .)