A number on a river (4)
I believe the answer is:
avon
'a river' is the definition.
(I've seen this before)
'a number on' is the wordplay.
'a number' becomes 'av' (I've seen this in other clues).
'av'+'on'='AVON'
(Other definitions for avon that I've seen before include "[WILTS] flower" , "River joining Bath with Bristol" , "River and former county" , "Bristol's river" , "Former English county" .)