It's used to catch small river fish (4)
I believe the answer is:
reel
'it's used' is the definition.
I can't judge whether this definition defines the answer.
'small river fish' is the wordplay.
'small river' becomes 'r' (abbreviation for river; 'small' asks for a short form).
'fish' becomes 'eel' (eel is a type of fish).
'r'+'eel'='REEL'
'to catch' acts as a link.
Can you help me to learn more?
(Other definitions for reel that I've seen before include "Stagger about in lively dance" , "Stagger around in the dance" , "energetic dance" , "Dance - sway - cotton holder" , "long stretch of film" .)