Fantastically pure English currency (5)
I believe the answer is:
rupee
'currency' is the definition.
(eg currency of India)
'fantastically pure english' is the wordplay.
'fantastically' indicates anagramming the letters (a fantastical ordering of the letters).
'english' becomes 'e' (abbreviation).
'pure' with letters rearranged gives 'rupe'.
'rupe'+'e'='RUPEE'
(Other definitions for rupee that I've seen before include "Indian currency unit" , "Currency of India, Pakistan and Bhutan" , "Indian monetary unit" , "Indian rhino" , "Indian money unit" .)