Imperial sovereign claiming victory, America anxious (7)
I believe the answer is:
nervous
'anxious' is the definition.
(synonyms)
'imperial sovereign claiming victory america' is the wordplay.
'imperial sovereign' becomes 'nero' (Roman emperor).
'claiming' is an insertion indicator.
'victory' becomes 'v'.
'america' becomes 'U.S.' (abbreviation for the United States).
'nero' enclosing 'v' is 'nervo'.
'nervo'+'us'='NERVOUS'
(Other definitions for nervous that I've seen before include "far from cool?" , "Anxious, uneasy" , "Anxious or apprehensive" , "Fidgety" , "Jumpy" .)