They swim off across river (5)
I believe the answer is:
krill
'they swim' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'off across river' is the wordplay.
'off' becomes 'kill' ('off' can informally mean 'murder').
'across' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'kill' enclosing 'r' is 'KRILL'.
Can you help me to learn more?
(Other definitions for krill that I've seen before include "Food of whalebone whales" , "Small shrimp-like creatures eaten by whales" , "Whales' staple diet" , "What baleen whales eat" , "Crustaceans, diet of whales" .)