Sit papers dealing with main god (8)
I believe the answer is:
Here is my best explanation:
'main god' is the definition.
(the main is an old nickname for the sea)
'sit papers dealing with' is the subsidiary indication.
'sit' becomes 'pose' ('pose' can be a synonym of 'sit').
'papers' becomes 'id' (identification papers).
'dealing with' becomes 'on'.
(Other definitions for poseidon that I've seen before include "Sea god", "being able to create waves?", "Greek god of the sea and earthquakes", "deity", "God", "Poisoned the Greek god of the sea", "Olympian", "associated with adventure on screen", "Scottish island" and "Controller of the main".)