It was a somewhat unsound currency (3)
I believe the answer is:
sou
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'a somewhat unsound currency' is the wordplay.
'a somewhat' indicates a hidden word.
'SOU' is hidden within 'unsound currency'.
'was' acts as a link.
Can you help me to learn more?
(Other definitions for sou that I've seen before include "Little money once" , "French bread once" , "Very small amount of money" , "Tiny amount of money" , "Old French monetary unit" .)