Antipodean city set down in a river (8)
I believe the answer is:
adelaide
'antipodean city' is the definition.
(I've seen this before)
'set down in a river' is the wordplay.
'set down' becomes 'laid' (I've seen this before).
'in' means one lot of letters goes inside another.
'river' becomes 'Dee' (River Dee).
'a'+'dee'='adee'
'laid' inserted into 'adee' is 'ADELAIDE'.
(Other definitions for adelaide that I've seen before include "Commonwealth city" , "19th century queen" , "Woman" , "Australian city named for William IV's queen" , "Australian capital" .)