Endless rain and snow upset swimmers (4)
I believe the answer is:
eels
'swimmers' is the definition.
(eel is a kind of aquatic vertebrate)
'endless rain and snow upset' is the wordplay.
'endless' means to remove the last letter.
'rain and snow' becomes 'sleet' (I've seen this before).
'upset' shows that the letters should be reversed in order.
'sleet' with its final letter taken away is 'slee'.
'slee' reversed gives 'EELS'.
(Other definitions for eels that I've seen before include "They swim" , "Abstract nineteen sixties style" , "Slippery creatures" , "(Slippery) fish" , "Long, fleshy fish" .)