Conservative slants overdue a fresh start (5,5)
I believe the answer is:
clean slate
'a fresh start' is the definition.
('clean slate' can be a synonym of 'fresh start')
'conservative slants overdue' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'slants' becomes 'leans' ('lean' can be a synonym of 'slant').
'overdue' becomes 'late' ('late' can be similar in meaning to 'overdue').
'c'+'leans'+'late'='CLEAN SLATE'
(Other definitions for clean slate that I've seen before include "good record" , "new start" , "A record without dishonour" , "making a fresh start?" .)