No end to the needles with the long swimmers (4)
I believe the answer is:
eels
'swimmers' is the definition.
(eel is a kind of aquatic vertebrate)
'no end to the needles with the long' is the wordplay.
'no' becomes 'e' (I am not sure about this - if you are sure you should believe this answer much more).
'end to' says to take the final letters.
'with' is an insertion indicator.
'the long' becomes 'l'.
The final letters of 'the needles' are 'es'.
'es' placed around 'l' is 'els'.
'e'+'els'='EELS'
Can you help me to learn more?
(Other definitions for eels that I've seen before include "Sea creatures found in lees" , "Lengthy fish" , "Long, fleshy fish" , "Fish (that may be jellied)" , "Jellied --, traditional dish" .)