Period in which doctrine is cast down (6)
I believe the answer is:
dismay
'cast down' is the definition.
('dismay' can be a synonym of 'cast down')
'period in which doctrine' is the wordplay.
'period' becomes 'day' (day is a kind of period).
'in which' means one lot of letters goes inside another.
'doctrine' becomes 'ism' (suffix for disciplines or schools of thought).
'day' enclosing 'ism' is 'DISMAY'.
'is' is the link.
(Other definitions for dismay that I've seen before include "Horrified amazement" , "Sadden" , "Feeling of despair in the face of obstacles" , "Fill with consternation" , "Disquiet or shock" .)