... cause pure chaos with European currency (5)
I believe the answer is:
rupee
'currency' is the definition.
(eg currency of India)
'cause pure chaos with european' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'pure' is 'rupe' which is present in the answer.
'european' could be 'e' (abbreviation e.g. EU) and 'e' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rupee that I've seen before include "Indian currency unit" , "Unit of money in India" , "Lots of pies" , "Indian rhino" , "Indian coin" .)