Split, one hundred remaining (5)
I believe the answer is:
cleft
'split' is the definition.
(synonyms)
'one hundred remaining' is the wordplay.
'one hundred' becomes 'c' (roman numeral).
'remaining' becomes 'left' (synonyms).
'c'+'left'='CLEFT'
(Other definitions for cleft that I've seen before include "Chin split" , "Fissure; divided" , "Opening, fissure" , "Split; fissure" , "Partially divided" .)