The quantity used in evening up (4)
I believe the answer is:
nine
'the quantity' is the definition.
(nine is a kind of quantity)
'in evening up' is the wordplay.
'in' indicates a hidden word.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'enin' is hidden in the letters of 'evening'.
'enin' reversed gives 'NINE'.
'used' is the link.
(Other definitions for nine that I've seen before include "Short time for a wonder" , "Number of the ancient Muses" , "A perfect square" , "So many" , "A number of worthies" .)