Quantity of archaism in the new (4)
I believe the answer is:
mint
'quantity' is the definition.
(mint is a kind of quantity)
'of archaism in the new' is the wordplay.
'of' indicates the answer is hidden within the clue.
'MINT' is hidden in the letters of 'archaism in the new'.
(Other definitions for mint that I've seen before include "Vast sum of money" , "strongroom in treasury" , "lots of money" , "but not used" , "Fragrant herb" .)