Sit papers dealing with main god (8)
I believe the answer is:
poseidon
'main god' is the definition.
(the main is an old nickname for the sea)
'sit papers dealing with' is the wordplay.
'sit' becomes 'pose' ('pose' can be a synonym of 'sit').
'papers' becomes 'id' (identification papers).
'dealing with' becomes 'on'.
'pose'+'id'+'on'='POSEIDON'
(Other definitions for poseidon that I've seen before include "Greek god of the sea and earthquakes" , "Controller of the main" , "deity" , "Scottish island" , "Poisoned the Greek god of the sea" .)