Loud performance, that's true (4)
I believe the answer is:
fact
'true' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'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'
'that's' is the link.
Can you help me to learn more?
(Other definitions for fact that I've seen before include "copyright protection agency?" , "Piece of information known to be true" , "Truth, reality" , "True item; reality" , "matter of record" .)