Foreign currency for exchange (6)
I believe the answer is:
rialto
'exchange' is the definition.
(I've seen this before)
'foreign currency for' is the wordplay.
'foreign currency' becomes 'rial' (currency of Iran).
'for' becomes 'to' (eg both can mean 'in the opinion of').
'rial'+'to'='RIALTO'
(Other definitions for rialto that I've seen before include "exchange" , "Old market" , "Dublin district, or Shakespeare's 'What news on the . . . . . .?'" , "Venice area with famous bridge" , "Island in Venice with famous bridge" .)