A French coin in Brazil? That’s illusory (6)
I believe the answer is:
unreal
'illusory' is the definition.
(similar in meaning)
'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' acts as a link.
(Other definitions for unreal that I've seen before include "Not actually existing" , "Existing only in the imagination, not actual" , "Lacking in substance" , "Fanciful" , "Phantasmagorical" .)