Weight of a vast rig at sea? (8)
I believe the answer is:
gravitas
'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).
'a'+'vast'+'rig'='avastrig'
'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" .)