Religious education is put in - fairly essential (9)
I believe the answer is:
requisite
'essential' is the definition.
(I know that essential can be written as requisite)
'religious education is put in fairly' is the wordplay.
'religious education' becomes 'RE' (abbreviation).
'put in' indicates putting letters inside.
'fairly' becomes 'quite' (I've seen this before).
'is' put within 'quite' is 'quisite'.
're'+'quisite'='REQUISITE'
(Other definitions for requisite that I've seen before include "Needful" , "Necessary" , "Indispensable" , "Totally essential" .)