Weight of rock (5)
I believe the answer is:
stone
I believe this is a double definition.
'weight' is the first definition.
(stone is a unit of weight)
'rock' is the second definition.
(I know that rock can be written as stone)
'of' is the link.
(Other definitions for stone that I've seen before include "Rock; fruit seed" , "14 lb" , "See 5" , "See 19" , "Rock; cherry seed" .)