Split when 100 resigned (5)

I believe the answer is:

cleft

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'split' is the definition.
(synonyms)

'100 resigned' is the wordplay.
'l00' becomes 'c' (Roman numeral).
'resigned' becomes 'left' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'c'+'left'='CLEFT'

'when' acts as a link.

Can you help me to learn more?

(Other definitions for cleft that I've seen before include "split along natural lines" , "Space" , "Fissure in rock" , "Chin split" , "Split; fissure" .)

I've seen this clue in The Irish Independent.

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