Mix-up involving plastic card, so ready money required (4,4)
I believe the answer is:
hard cash
'ready money required' is the definition.
(ready money is hard cash)
'mix-up involving plastic card' is the wordplay.
'mix up' becomes 'hash' (making a hash of something is messing it up).
'involving' indicates putting letters inside.
'plastic' indicates anagramming the letters (plastic can mean easily moulded).
'card' anagrammed gives 'rdca'.
'hash' placed around 'rdca' is 'HARD CASH'.
'so' is the link.
(Other definitions for hard cash that I've seen before include "Not plastic!" , "Coins and notes, not cheques or promises" , "Coins and banknotes - not a cheque" , "Ready money" .)