Modern currency? Scream 'opt out!' (6)
I believe the answer is:
crypto
'modern currency?' is the definition.
(crypto-currency)
'scream opt out' is the wordplay.
'scream' becomes 'cry' (synonyms).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'opt' with letters rearranged gives 'pto'.
'cry'+'pto'='CRYPTO'
(Other definitions for crypto that I've seen before include "closet" , "Prefix denoting hidden or obscure" .)