A small number do weights endlessly (5)
I believe the answer is:
eight
'a small number do' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'weights endlessly' is the wordplay.
'endlessly' says to take the centre (the ends of the words are removed).
The middle letters of 'weights' are 'EIGHT'.
Can you help me to learn more?
(Other definitions for eight that I've seen before include "power of two" , "No. of stones in cwt." , "Oxygen's atomic number" , "Figure" , "Number etched by a skater?" .)