River god: in France he principally inhabits bell towers (9)
I believe the answer is:
campanili
'bell towers' is the definition.
(I've seen this in another clue)
'river god in france he principally inhabits' is the wordplay.
'river' becomes 'cam' (river in Cambridge, England).
'god' becomes 'Pan' (Pan is a Greek god).
'in france he' becomes 'il' ('he' in French).
'principally' says to take the initial letters.
The first letter of 'inhabits' is 'i'.
'cam'+'pan'+'il'+'i'='CAMPANILI'
(Other definitions for campanili that I've seen before include "towers required" , "bell-towers" .)