Fateful day with teams lacking leader (4)
I believe the answer is:
ides
'fateful day' is the definition.
(Julius Caesar was killed on the ides of March)
'teams lacking leader' is the wordplay.
'teams' becomes 'sides' (eg football sides).
'lacking leader' means to remove the first letter.
'sides' with its initial letter taken off is 'IDES'.
'with' is the link.
(Other definitions for ides that I've seen before include "15/7?" , "Mid-month day in the Roman calendar" , "One da" , "Fatal March date or Julius Caesar" , "For Romans, a day" .)