Certain pressure before I vote is wrong (8)
I believe the answer is:
positive
'certain' is the definition.
('positive' can be similar in meaning to 'certain')
'pressure before i vote is wrong' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'before' says to put letters next to each other.
'wrong' indicates anagramming the letters.
'i'+'vote'+'is'='ivoteis'
'ivoteis' with letters rearranged gives 'ositive'.
'p'+'ositive'='POSITIVE'
(Other definitions for positive that I've seen before include "Confident, very sure" , "'Definite, categorical (8)'" , "Optimistic; certain" , "Certain, confident in opinion" , "Of a number, greater than zero - that's certain" .)