A French coin in Brazil? That’s fantastic (6)
I believe the answer is:
unreal
'fantastic' is the definition.
(fantastic can mean unreal or imaginary)
'a french coin in brazil?' is the wordplay.
'a french' becomes 'un' ('a' in French).
'coin in brazil?' becomes 'real' (real is the currency of Brazil).
'un'+'real'='UNREAL'
'that's' is the link.
(Other definitions for unreal that I've seen before include "Not actual or credible" , "Not actually existing" , "Artificial, insubstantial" , "Not factual" , "Fanciful" .)