Shrimplike creatures do in surrounding river (5)
I believe the answer is:
krill
'shrimplike creatures' is the definition.
(I've seen this before)
'do in surrounding river' is the wordplay.
'do in' becomes 'kill' (do in can mean to assassinate or kill).
'surrounding' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'kill' going around 'r' is 'KRILL'.
(Other definitions for krill that I've seen before include "What baleen whales eat" , "seafood" , "Whale food" , "Tiny shrimps" , "Crustaceans eaten by whales" .)