Weight of stone (3)
I believe the answer is:
ton
'weight' is the definition.
(ton is a kind of weight)
'of stone' is the wordplay.
'of' says the answer is hidden in the clue.
'TON' is hidden in the letters of 'stone'.
(Other definitions for ton that I've seen before include "A slang 100 mph" , "2240 Ibs" , "Two thousand - two hundred and forty pounds" , "20 cwt" , "Fashionable" .)