Fish begin endlessly entertaining lethargic swim (4)
I believe the answer is:
eels
'fish' is the definition.
(I've seen this before)
'begin endlessly entertaining lethargic swim' is the wordplay.
'begin' indicates taking the first letters.
The initial letters of 'endlessly entertaining lethargic swim' is 'EELS'.
(Other definitions for eels that I've seen before include "Long fishes" , "Slender fish" , "Jellied --, traditional dish" , "Smooth fishes" , "They have long bodies and very small fins" .)