Spear fish in river (7)
I believe the answer is:
trident
'spear fish in river' is the definition.
'trident' can be an answer for 'spear' (I've seen this before). I am not certain of the 'fish in river' bit.
'spear fish in river' is the wordplay.
'spear fish' becomes 'id' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'river' becomes 'trent' (River Trent).
'id' inserted inside 'trent' is 'TRIDENT'.
Can you help me to learn more?
(Other definitions for trident that I've seen before include "3-pronged spear" , "Submarine-launched ballistic missile" , "Nuclear missile" , "Submarine missile" , "not pike" .)