That is the day for assassination, foremost of tragedies (2,3)
I believe the answer is:
id est
'that is' is the definition.
('that is' in Latin)
'the day for assassination foremost of tragedies' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'day' could be 'ides' (ides is a kind of day) and 'ides' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for id est that I've seen before include "Ie, in full" , "In other words" , "Namely" , "That is to say (Latin)" , "I mean to say!" .)