Weight of boulder (5)
I believe the answer is:
stone
I believe this is a double definition.
'weight' is the first definition.
(I've seen this before)
'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 "Rock; cherry seed" , "Little rock" , "Olive pip" , "Boulder" , "Rock; fruit seed" .)