Number by river meeting arts benefactor, a big noise (9)
I believe the answer is:
potentate
'a big noise' is the definition.
(type of ruler - big noise means someone important)
'number by river meeting arts benefactor' is the wordplay.
'number' becomes 'ten' (ten is a kind of whole number).
'by' means one lot of letters go next to another.
'river' becomes 'Po' (Italian river).
'meeting' is a charade indicator (letters next to each other).
'arts benefactor' becomes 'Tate' (as in the Tate art galleries).
'ten' after 'po' is 'poten'.
'poten'+'tate'='POTENTATE'
(Other definitions for potentate that I've seen before include "authority" , "A monarch or ruler" , "Mogul" , "Powerful autocratic ruler" .)