Weight of a boulder (5)
I believe the answer is:
I believe this is a double definition.
'weight' is the first definition.
(stone is a kind of weight)
'a boulder' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for stone that I've seen before include "Olive pip" , "See 21" , "diamond, say" , "Rock; gem" , "Boulder" .)