Excitement? I'm in a state (9)
I believe the answer is:
animation
'excitement?' is the definition.
(thesaurus)
'i'm in a state' is the wordplay.
'in' means one lot of letters goes inside another.
'state' becomes 'nation' ('nation' can be a synonym of 'state'**).
'a'+'nation'='anation'
'im' inserted within 'anation' is 'ANIMATION'.
(Other definitions for animation that I've seen before include "Liveliness and vigour" , "Cartoon-making art" , "Liveliness and enthusiasm" , "Liveliness - illusion of movement in cartoon films" , "(State of) being full of life" .)