A fateful day for military assistants (5)
I believe the answer is:
aides
'military assistants' is the definition.
(I've seen this before)
'a fateful day' is the wordplay.
'fateful day' becomes 'ides' (Julius Caesar was killed on the ides of March).
'a'+'ides'='AIDES'
'for' acts as a link.
(Other definitions for aides that I've seen before include "Helpers" , "Ministerial assistants" , "Adjutants" , "Military assistants to a senior officers" , "Confidential assistants" .)