Number of weight-lifters? (5)
I believe the answer is:
eight
'number' is the definition.
(eight is a kind of number)
'of weight-lifters?' is the wordplay.
'of' indicates the answer is hidden within the clue.
'EIGHT' is hidden within 'weight lifters'.
(Other definitions for eight that I've seen before include "may be cut out of ice" , "Mathematical cube" , "One less than nineteen across" , "Hit, e.g. (anag.)" , "23" .)