Cathedral city on river bank (4)
I believe the answer is:
rely
'bank' is the definition.
(thesaurus)
'cathedral city on river' is the wordplay.
'cathedral city' becomes 'ely' (I've seen this before**).
'on' means one lot of letters go next to another.
'river' becomes 'r'.
'ely' after 'r' is 'RELY'.
(Other definitions for rely that I've seen before include "City with cathedral" , "Be dependent" , "Be sure" , "All the letters" , "Lyre (anag.)" .)