Excitement as I'm entering American state (9)
I believe the answer is:
animation
'excitement' is the definition.
(thesaurus)
'i'm entering american state' is the wordplay.
'entering' is an insertion indicator.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'state' becomes 'nation' ('nation' can be a synonym of 'state'**).
'a'+'nation'='anation'
'im' placed within 'anation' is 'ANIMATION'.
'as' is the link.
(Other definitions for animation that I've seen before include "Liveliness and enthusiasm" , "Being full of energy" , "Liveliness and vigour" , "See 6" , "Cartoon-making art" .)