Immediately and with fatal consequences holding court (8)
I believe the answer is:
directly
'immediately' is the definition.
('directly' can be a synonym of 'immediately')
'with fatal consequences holding court' is the wordplay.
'with fatal consequences' becomes 'direly'.
'holding' means one lot of letters goes inside another.
'court' becomes 'ct'.
'direly' enclosing 'ct' is 'DIRECTLY'.
'and' acts as a link.
(Other definitions for directly that I've seen before include "Immediately or without deviation" , "In a straightforward manner, no delay" , "following the shortest route possible?" , "As the crow flies" , "in a trice!" .)