An opinion to contradict loudly (6)
I believe the answer is:
belief
'an opinion' is the definition.
(an opinion is a personal belief)
'contradict loudly' is the wordplay.
'contradict' becomes 'belie' (synonyms).
'loudly' becomes 'f' (short for forte - 'loud' in music).
'belie'+'f'='BELIEF'
'to' acts as a link.
(Other definitions for belief that I've seen before include "Religious conviction" , "Persuasion" , "Faith, doctrine" , "Faith, credence" , "Conviction, faith" .)