Complaint about fake news possibly reduced trust (6)
I believe the answer is:
belief
'trust' is the definition.
(I've seen this before)
'complaint about fake news possibly reduced' is the wordplay.
'complaint' becomes 'beef' (I've seen this before**).
'about' means one lot of letters goes inside another.
'fake news possibly' becomes 'lie'.
'reduced' means to remove the last letter.
'lie' with its last letter taken away is 'li'.
'beef' enclosing 'li' is 'BELIEF'.
(Other definitions for belief that I've seen before include "Religious conviction" , "Principle accepted without proof" , "Tenet, creed" , "Conviction, faith" , "Creed or opinion" .)