A strong line to get fish out of the river (4)
I believe the answer is:
cord
'a strong line' is the definition.
'cord' can be an answer for 'line' (cord is a kind of line). I am not sure about the 'a strong' bit.
'fish out of the river' is the wordplay.
'fish' becomes 'cod' (type of fish).
'out of' means one lot of letters goes inside another (some letters on the outside of others).
'the river' becomes 'r' (abbreviation for river).
'cod' placed around 'r' is 'CORD'.
'to get' is the link.
Can you help me to learn more?
(Other definitions for cord that I've seen before include "A measure of cut wood" , "Strong string" , "Thin string" , "Thin rope" , "Small rope; flex" .)