Weight of stone (3)
I believe the answer is:
ton
'weight' is the definition.
(unit of weight)
'of stone' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TON' is hidden within '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" .)