Religious Education most important, don’t leave (6)
I believe the answer is:
remain
'don't leave' is the definition.
(remaining is the opposite of leaving)
'religious education most important' is the wordplay.
'religious education' becomes 'RE' (abbreviation).
'most important' becomes 'main' (main can mean the most important or major).
're'+'main'='REMAIN'
(Other definitions for remain that I've seen before include "Stay (in a place)" , "Continue in position" , "Stay behind" , "Stay on when others have left" , "Don't leave (the EU?)" .)