State capital placed within a river (8)
I believe the answer is:
'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).
'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" .)