Joke made by working class about Labour’s leader (10)
I believe the answer is:
pleasantry
'joke' is the definition.
(I know that pleasantry is a type of joke)
'working class about labour's leader' is the wordplay.
'working class' becomes 'peasantry'.
'about' means one lot of letters goes inside another.
'leader' indicates taking the first letters.
The initial letter of 'labours' is 'l'.
'peasantry' going around 'l' is 'PLEASANTRY'.
'made by' is the link.
(Other definitions for pleasantry that I've seen before include "Source of laughter" , "facetious trick" , "Joke, banter" , "Polite remark" , "Slightly amusing remark" .)