Conservative in late spoiled striking effect (5)
I believe the answer is:
eclat
'striking effect' is the definition.
(I've seen this before)
'conservative in late spoiled' is the wordplay.
'conservative' becomes 'C' (abbreviation commonly used in UK politics).
'in' indicates putting letters inside.
'spoiled' is an anagram indicator.
'late' is an anagram of 'elat'.
'c' going within 'elat' is 'ECLAT'.
(Other definitions for eclat that I've seen before include "Enthusiastic approval" , "brilliant effort" , "Ostentation" , "Brilliant display -- conspicuous success" , "Brilliance" .)