Weight of a vast rig collapsing (8)
I believe the answer is:
gravitas
'weight' is the definition.
(I've seen this before)
'a vast rig collapsing' is the wordplay.
'collapsing' is an anagram indicator.
'a'+'vast'+'rig'='avastrig'
'avastrig' is an anagram of 'GRAVITAS'.
'of' is the link.
(Other definitions for gravitas that I've seen before include "Dignity; air of seriousness" , "Seriousness - importance" , "Dignified manner/presence" , "Seriousness - sobriety" , "Dignity, solemnity" .)