State capital placed within a river (8)
I believe the answer is:
adelaide
'state capital' is the definition.
(state capital of South Australia)
'placed within a river' is the wordplay.
'placed' becomes 'laid' ('lay' can be a synonym of 'place').
'within' is an insertion indicator.
'river' becomes 'Dee' (River Dee).
'a'+'dee'='adee'
'laid' placed inside 'adee' is 'ADELAIDE'.
(Other definitions for adelaide that I've seen before include "cook turkey on the barbie here?" , "Lead aide out to Australia" , "State capital of South Australia" , "Antipodean city" , "Lead aide to Oz" .)