Incredulous utterance, Anglo-Saxon, loud: Everyman's entertained (2,2)
I believe the answer is:
as if
'incredulous utterance' is the definition.
(shows disbelief)
'anglo-saxon loud everyman's entertained' is the wordplay.
'anglo-saxon' becomes 'AS' (abbreviation for Anglo-Saxon).
'loud' becomes 'f' (short for forte - 'loud' in music).
'everyman' becomes 'i' (where the crossword setter is 'Everyman').
'entertained' indicates putting letters inside.
'as'+'f'='asf'
'asf' going around 'i' is 'AS IF'.
(Other definitions for as if that I've seen before include "highly unlikely" , "Seemingly" , "Expression of incredulity" , "Or though" , "Yeah right!" .)