Neither number’s never-ending! (3)
I believe the answer is:
nor
'neither' is the definition.
(I've seen this before)
'number's never-ending' is the wordplay.
'number' becomes 'no' (abbreviation).
'ending' suggests the final letters.
The final letter of 'never' is 'r'.
'no'+'r'='NOR'
(Other definitions for nor that I've seen before include "Conjunction meaning 'and not'" , "And not either" , "Neither pairing" , "Word paired with "neither"" , "logic circuit" .)