Tremendous performance opposed by Conservative (8)
I believe the answer is:
gigantic
'tremendous' is the definition.
(synonyms)
'performance opposed by conservative' is the wordplay.
'performance' becomes 'gig' (a gig is a musical concert).
'opposed' becomes 'anti' (similar in meaning).
'by' means one lot of letters go next to another.
'conservative' becomes 'C' (UK political abbreviation).
'gig'+'anti'+'c'='GIGANTIC'
(Other definitions for gigantic that I've seen before include "Titanic" , "monster" , "walloping" , "Exceedingly large" , "Huge, enormous" .)