A quantity of plutonium (3)
I believe the answer is:
ton
'a 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 "20 cwt" , "A slangy 100 mph - heavy" , "A slang 100mph - heavy" , "Two thousand - two hundred and forty pounds" , "Weight - fashion (French)" .)