Believe spade for one to be plastic (6,4)
I believe the answer is:
credit card
'be plastic' is the definition.
(I've seen this before)
'believe spade for one' is the wordplay.
'believe' becomes 'credit' (I've seen this before).
'spade for one' becomes 'card' (spades is a playing card suit).
'credit'+'card'='CREDIT CARD'
'to' is the link.
(Other definitions for credit card that I've seen before include "Means of borrowing" , "Piece of "plastic''" , "Entrant to 1 Across [CASH DISPENSER]" .)