French currency sounds reasonable (5)
I believe the answer is:
'french currency' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'sounds reasonable' is the wordplay.
'sounds' indicates a 'sounds like' (homophone) clue.
'reasonable' becomes 'sane' (similar in meaning).
'sane' sounds like 'SEINE'.
Can you help me to learn more?
(Other definitions for seine that I've seen before include "Paris's river" , "Second longest river in France" , "Paris river" , "Parisian river" , "French river -- type of fishing net" .)