Dead good with United (4)
I believe the answer is:
gone
'dead' is the definition.
(similar in meaning)
'good with united' is the wordplay.
'good' becomes 'g' (abbreviation).
'with' means one lot of letters go next to another.
'united' becomes 'one' (as in a phrase like 'we are one').
'g'+'one'='GONE'
(Other definitions for gone that I've seen before include "... saga of 2's civil war" , "Departed; used up" , "Moved - died" , "Departed, left" , "Away, departed" .)