Conservative lists formerly unblemished record (5,5)
I believe the answer is:
clean slate
'unblemished record' is the definition.
(a clean slate is a record with no faults recorded)
'conservative lists formerly' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'lists' becomes 'leans' (I've seen this in another clue).
'formerly' becomes 'late' (I've seen this in other clues).
'c'+'leans'+'late'='CLEAN SLATE'
(Other definitions for clean slate that I've seen before include "A record without dishonour" , "making a fresh start?" , "good record" , "new start" .)