Roman god who was enthroned on a vessel (6)
I believe the answer is:
saturn
'roman god who' is the definition.
'Saturn' can be an answer for 'roman god' (Saturn is an example). I'm unsure of the remainder of the definition.
'enthroned on a vessel' is the wordplay.
'enthroned on' becomes 'sat' (I am not sure about this - if you are sure you should believe this answer much more).
'a vessel' becomes 'urn' (urn is a type of container).
'sat'+'urn'='SATURN'
'was' is the link.
Can you help me to learn more?
(Other definitions for saturn that I've seen before include "One of the 1 across" , "Planet with concentric rings" , "Sixth planet from the sun" , "deity" , "Giant planet" .)