Time to suppress belief and sadness (6)
I believe the answer is:
dismay
'sadness' is the definition.
(dismay can mean sadness or concern)
'time to suppress belief' is the wordplay.
'time' becomes 'day' (day is a kind of time).
'to suppress' indicates putting letters inside.
'belief' becomes 'ism' (the -ism suffix in the names of belief systems).
'day' placed around 'ism' is 'DISMAY'.
'and' is the link.
(Other definitions for dismay that I've seen before include "Alarm, apprehension" , "Appal" , "Concern and distress at something unexpected" , "Horrified amazement" , "Fill with apprehension" .)