Aquatic creatures from river involved in slaughter (5)
I believe the answer is:
krill
'aquatic creatures' is the definition.
'krill' can be an answer for 'creatures' (I have seen 'Shrimplike creatures' mean 'krill' so perhaps 'creatures' could also mean 'krill'). I'm unsure of the 'aquatic' bit.
'river involved in slaughter' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'involved in' is an insertion indicator.
'slaughter' becomes 'kill' (I've seen this before).
'r' going inside 'kill' is 'KRILL'.
'from' is the link.
Can you help me to learn more?
(Other definitions for krill that I've seen before include "Small shrimp-like creatures eaten by whales" , "Small aquatic creatures" , "Very small crustaceans" , "Tiny crustaceans" , "Food of whalebone whales" .)