Never retired — a hundred! (3)
I believe the answer is:
ton
'a hundred' is the definition.
(I've seen this before)
'never retired' is the wordplay.
'never' becomes 'not' (I've seen this in other clues).
'retired' is a reversal indicator.
'not' backwards is 'TON'.
(Other definitions for ton that I've seen before include "Heavyweight" , "People of fashion" , "Speed" , "2,240 lbs" , "Unit of weight equivalent to 2240 pounds" .)