Everyone that is outside can upset union (8)
I believe the answer is:
alliance
'union' is the definition.
(alliance is a kind of union)
'everyone that is outside can upset' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'outside' is an insertion indicator.
'upset' is an anagram indicator.
'can' anagrammed gives 'anc'.
'ie' placed around 'anc' is 'iance'.
'all'+'iance'='ALLIANCE'
(Other definitions for alliance that I've seen before include "Association for mutual benefit" , "Formal agreement" , "A joining of forces" , "Federation" , "Kind of partnership with canaille" .)