Weight of a vast rig at sea? (8)
I believe the answer is:
'weight' is the definition.
(I've seen this before)
'a vast rig at sea?' is the wordplay.
'at sea?' indicates an anagram (I've seen 'at sea' mean this).
'avastrig' with letters rearranged gives 'GRAVITAS'.
'of' is the link.
(Other definitions for gravitas that I've seen before include "serious quality" , "Dignity; air of seriousness" , "Dignity, solemnity" , "Seriousness - importance" , "Solemnity of manner" .)