Opinion to contradict loudly (6)
I believe the answer is:
belief
'opinion' is the definition.
(thesaurus)
'contradict loudly' is the wordplay.
'contradict' becomes 'belie' ('belie' can be a synonym of 'contradict').
'loudly' becomes 'f' (A musical term).
'belie'+'f'='BELIEF'
'to' acts as a link.
(Other definitions for belief that I've seen before include "Creed or opinion" , "Conviction, faith" , "Faith, doctrine" , "Trust, faith (6)" , "Faith, credence" .)