In river I had Poseidon's spear (7)
I believe the answer is:
trident
'poseidon's spear' is the definition.
(I've seen this in another clue)
'in river i had' is the wordplay.
'in' is an insertion indicator.
'river' becomes 'trent' (River Trent).
'i had' becomes 'I'd' (I'd).
'trent' placed around 'id' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Red tint? (anagram)" , "Red tint (anag.)" , "Neptune's or Britannia's spear" , "not pike" , "Nuclear weapon" .)