Bell towers river god observed on the Italian island (9)
I believe the answer is:
campanili
'bell towers' is the definition.
(I've seen this in another clue)
'river god observed on the italian island' is the wordplay.
'river' becomes 'cam' (river in Cambridge, England).
'god' becomes 'Pan' (Pan is a Greek god).
'observed on' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'the italian' becomes 'il' ('the' in Italian).
'island' becomes 'i' (geographical abbreviation).
'cam'+'pan'+'il'+'i'='CAMPANILI'
(Other definitions for campanili that I've seen before include "towers required" , "bell-towers" .)