Almost frighten one Conservative? Wicked! (8)
I believe the answer is:
terrific
'wicked' is the definition.
(I've seen this before)
'almost frighten one conservative?' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'frighten' becomes 'terrify' (terrifying is a kind of frightening).
'one' becomes 'i' (Roman numeral).
'conservative?' becomes 'C' (UK political abbreviation).
'terrify' with its final letter removed is 'terrif'.
'terrif'+'i'+'c'='TERRIFIC'
(Other definitions for terrific that I've seen before include "Magnificent" , "wicked" , "Very intense" , "Excellent - awe-inspiring" , "frightening" .)