Fish doze endlessly going round (4)
I believe the answer is:
eels
'fish' is the definition.
(I've seen this before)
'doze endlessly going round' is the wordplay.
'doze' becomes 'sleep' (I've seen this before).
'endlessly' means to remove the last letter.
'going round' shows that the letters should be reversed in order.
'sleep' with its last letter taken away is 'slee'.
'slee' reversed gives 'EELS'.
(Other definitions for eels that I've seen before include "Slippery types" , "Snakes wriggling in lees" , "Congers" , "Odd fish found in the lees" , "Smooth fishes" .)