Tremendous performance against Conservative (8)
I believe the answer is:
gigantic
'tremendous' is the definition.
('gigantic' can be a synonym of 'tremendous')
'performance against conservative' is the wordplay.
'performance' becomes 'gig' (I've seen this before).
'against' becomes 'anti' (I've seen this before).
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'gig'+'anti'+'c'='GIGANTIC'
(Other definitions for gigantic that I've seen before include "Stupendous" , "Fantastically great" , "Great big" , "walloping" , "Titanic" .)