Loud performance, it's true (4)
I believe the answer is:
fact
'it's true' is the definition.
(I've seen this in another clue)
'loud performance' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'performance' becomes 'act' (act is a kind of performance).
'f'+'act'='FACT'
(Other definitions for fact that I've seen before include "matter of record" , "Not fake news!" , "It's certainly not fiction" , "Datum" , "Something known to be true" .)