Quantity of plutonium (3)
I believe the answer is:
ton
'quantity' is the definition.
(ton is a kind of quantity)
'of plutonium' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TON' is hidden in the letters of 'plutonium'.
(Other definitions for ton that I've seen before include "A slang 100mph - heavy" , "Twenty cwt" , "U nit of weight" , "See 25" , "100 mph, runs, etc" .)