Fountain: priest thrown into it backwards (5)
I believe the answer is:
trevi
'fountain' is the definition.
(I've seen this before)
'priest thrown into it backwards' is the wordplay.
'priest' becomes 'rev' (short for reverend).
'thrown into' is an insertion indicator.
'backwards' shows that the letters should be reversed in order.
'it' back-to-front is 'ti'.
'rev' put within 'ti' is 'TREVI'.
(Other definitions for trevi that I've seen before include "flowing in Rome" , "Roman fountain" , "Rome fountain" .)