Place religious education before science? Not I. Rot! (11)
I believe the answer is:
putrescence
'rot' is the definition.
(putrescence is a kind of rot)
'place religious education before science? not i' is the wordplay.
'place' becomes 'put' ('put' can be a synonym of 'place'**).
'religious education' becomes 're'.
'before' means one lot of letters go next to another.
'not' indicates named letters should be taken away.
'science' with 'i' removed is 'scence'.
'put'+'re'+'scence'='PUTRESCENCE'
(Other definitions for putrescence that I've seen before include "Rot!" , "Decay, rottenness" .)