Past Socialist turning Conservative becomes offensive (10)
I believe the answer is:
derogatory
'offensive' is the definition.
(I've seen this before)
'past socialist turning conservative' is the wordplay.
'past' becomes 'ago' (similar in meaning).
'socialist' becomes 'red' (socialism is often associated with the colour red).
'turning' shows that the letters should be reversed in order.
'conservative' becomes 'tory' (Tory means a member of the Conservative Party).
'ago'+'red'='agored'
'agored' backwards is 'deroga'.
'deroga'+'tory'='DEROGATORY'
'becomes' acts as a link.
(Other definitions for derogatory that I've seen before include "Showing a disrespectful attitude" , "deliberately offensive" , "Insulting" , "Disparaging" .)