Most immediate gain overwhelming god (7)
I believe the answer is:
nearest
'most immediate' is the definition.
(immediate can mean close by)
'gain overwhelming god' is the wordplay.
'gain' becomes 'net' (net is a kind of gain).
'overwhelming' means one lot of letters goes inside another.
'god' becomes 'ares' (Greek god of war).
'net' placed around 'ares' is 'NEAREST'.
(Other definitions for nearest that I've seen before include "In greatest propinquity" , "Eastern (anag)" , "Relations" , "Most accessible" , "most like Scrooge at first" .)