Pleasure -- it has returned in Conservative Party? (8)
I believe the answer is:
function
'party?' is the definition.
(thesaurus)
'pleasure it has returned in conservative' is the wordplay.
'pleasure' becomes 'fun' (I've seen this before).
'has returned' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'conservative' becomes 'con' (abbreviation for Conservative).
'it' reversed gives 'ti'.
'ti' placed within 'con' is 'ction'.
'fun'+'ction'='FUNCTION'
(Other definitions for function that I've seen before include "Role, duty" , "Special purpose or social affair" , "An official ceremony" , "Large formal event" , "Operate" .)