One-fifth of 100 (6)
I believe the answer is:
twenty
'one-fifth of 100' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for twenty that I've seen before include "Hundredweights to a ton" , "made by Q on a light blue square" , "Score, XX" , "Figure" , "Dozen plus eight" .)