Old fiddle turned up in Brazilian city's river (7)
I believe the answer is:
orinoco
'river' is the definition.
(I know that Orinoco is a river)
'old fiddle turned up in brazilian city's' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'fiddle' becomes 'con' (both can mean a scam).
'turned up' is a reversal indicator.
'in' indicates putting letters inside.
'brazilian city' becomes 'Rio' (Rio de Janeiro).
'con' back-to-front is 'noc'.
'noc' put into 'rio' is 'rinoco'.
'o'+'rinoco'='ORINOCO'
(Other definitions for orinoco that I've seen before include "Colombia/Venezuela river" , "South-American river" , "South American river flowing into the Atlantic" , "Great river flowing through Venezuela to Atlantic" , "may be seen running through South America" .)