Sea-god's spear I had found in river (7)
I believe the answer is:
trident
'sea-god's spear' is the definition.
'trident' can be an answer for 'spear' (trident is a kind of spear). I am unsure of the 'sea-god's' bit.
'i had found in river' is the wordplay.
'i had' becomes 'i'd' (contraction).
'found in' indicates putting letters inside.
'river' becomes 'trent' (River Trent).
'id' inserted within 'trent' is 'TRIDENT'.
Can you help me to learn more?
(Other definitions for trident that I've seen before include "3-pronged spear" , "Three-pronged weapon" , "Britannia's weapon" , "Red tint (anag.)" , "Nuclear weapon" .)