The best Religious Education received in academic stream (5)
I believe the answer is:
cream
'the best' is the definition.
(I've seen this before)
'religious education received in academic stream' is the wordplay.
'religious education' becomes 're'.
'received in' is an insertion indicator.
'academic stream' becomes 'cam' (River cam runs through Cambridge).
're' going into 'cam' is 'CREAM'.
(Other definitions for cream that I've seen before include "An off-white shade" , "Elite; pale yellow" , "Butterfat part of milk" , "Thick milk" , "Fat-rich part of milk" .)