It's on the cards as being merely plastic (6)
I believe the answer is:
credit
'merely plastic' is the definition.
'credit' can be an answer for 'plastic' (I've seen this in another clue). I'm not sure about the 'merely' bit.
'it's on the cards as' is the wordplay.
'on' is a charade indicator (letters next to each other).
'the cards as' becomes 'cred' (I can't explain this - if you can you should give a lot more credence to this answer).
'it' put after 'cred' is 'CREDIT'.
'being' is the link.
Can you help me to learn more?
(Other definitions for credit that I've seen before include "If this is good, you can borrow money" , "Title" , "Source of pride; account entry" , "Have faith" , "Believe the advance of money" .)