Caligula’s so cruel, almost to the end (3)
I believe the answer is:
sic
'caligula's so' is the definition.
('so' in Latin - Caligula was a Roman emperor)
'cruel almost to the end' is the wordplay.
'cruel' becomes 'sick' (sick can mean cruel or sadistic).
'almost to the end' means to remove the last letter (most but not all of the word).
'sick' with its last letter removed is 'SIC'.
(Other definitions for sic that I've seen before include "poorly spelt if requiring this" , "Thus in Corsica" , "Printing term to denote accuracy though it appears otherwise" , "Intentionally so written" , "(Of quoted word) exactly so" .)