Disparaging past socialist turned Conservative (10)
I believe the answer is:
derogatory
'disparaging' is the definition.
('derogatory' can be a synonym of 'disparaging')
'past socialist turned conservative' is the wordplay.
'past' becomes 'ago' ('ago' can be similar in meaning to 'past').
'socialist' becomes 'red' (red is a kind of socialist).
'turned' is a reversal indicator.
'conservative' becomes 'tory' (I've seen this before).
'ago'+'red'='agored'
'agored' in reverse letter order is 'deroga'.
'deroga'+'tory'='DEROGATORY'
(Other definitions for derogatory that I've seen before include "deliberately offensive" , "Showing a disrespectful attitude" , "Insulting" , "Disparaging" .)