Grumble about the Italian switching faith (6)
I believe the answer is:
belief
'faith' is the definition.
(I've seen this before)
'grumble about the italian switching' is the wordplay.
'grumble' becomes 'beef' (both can mean to complain).
'about' indicates putting letters inside.
'the italian' becomes 'il' ('the' in Italian).
'switching' says the letters should be written backwards (switching direction).
'il' reversed gives 'li'.
'beef' placed around 'li' is 'BELIEF'.
(Other definitions for belief that I've seen before include "Tenet, creed" , "Creed or opinion" , "Credo" , "Persuasion" , "Conviction, faith" .)