100 years — not half clever age (5)
I believe the answer is:
cycle
'age' is the definition.
(I've seen this in another clue)
'100 years not half clever' is the wordplay.
'100' becomes 'C' (Roman numeral).
'years' becomes 'y' (abbreviation).
'not half' means to take half the letters.
'clever' cut in half is 'cle'.
'c'+'y'+'cle'='CYCLE'
(Other definitions for cycle that I've seen before include "Sequence of events that repeats" , "Recurring sequence" , "Recurring set of events" , "Recurrent series" , "Recurring series of events" .)