Loud turn, in reality (4)
I believe the answer is:
fact
'in reality' is the definition.
(thesaurus)
'loud turn' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'turn' becomes 'act' (theatre).
'f'+'act'='FACT'
(Other definitions for fact that I've seen before include "Piece of indisputable information" , "Fait accompli" , "given" , "Statement of verifiable information" , "Certain thing" .)