In truth a loud performance (4)
I believe the answer is:
fact
'in truth' is the definition.
(I've seen this before)
'a loud performance' is the wordplay.
'a 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 "Datum" , "Certain thing" , "Verifiable truth" , "Undisputed truth" , "matter of record" .)