River running by state capital (7)
I believe the answer is:
trenton
'state capital' is the definition.
(Trenton is an example)
'river running by' is the wordplay.
'river' becomes 'trent' (I've seen this before**).
'running' becomes 'on' (I've seen this in other clues).
'by' means one lot of letters go next to another.
'trent'+'on'='TRENTON'
(Other definitions for trenton that I've seen before include "Seventeen's [NEW JERSEY] capital" , "New Jersey city" , "NJ city" , "US city" , "State capital of New Jersey" .)