Old money has an honest sound (5)
I believe the answer is:
franc
'old money' is the definition.
(I've seen this before)
'an honest sound' is the wordplay.
'an honest' becomes 'frank' (I've seen this in another clue).
'sound' indicates a 'sounds like' (homophone) clue.
'frank' sounds like 'FRANC'.
'has' acts as a link.
(Other definitions for franc that I've seen before include "Swiss loot" , "Foreign capital" , "It's equal to 100 centimes" , "I'm accepted in Basle" , "European currency until the end of 2001" .)