Not making a century (3)
I believe the answer is:
ton
'a century' is the definition.
(both can mean one hundred of something)
'not making' is the wordplay.
'making' indicates anagramming the letters.
'not' is an anagram of 'TON'.
(Other definitions for ton that I've seen before include "Two thousand, two hundred and forty lbs avoirdupois" , "Heavy weight or 100mph" , "Two thousand - two hundred and forty pounds" , "A slangy 100mph - heavy going" , "20cwt" .)