I'm in a state -- it's excitement (9)
I believe the answer is:
animation
'excitement' is the definition.
(thesaurus)
'i'm in a state' is the wordplay.
'in' is an insertion indicator.
'state' becomes 'nation' ('nation' can be a synonym of 'state'**).
'a'+'nation'='anation'
'im' going inside 'anation' is 'ANIMATION'.
'it's' acts as a link.
(Other definitions for animation that I've seen before include "Vigour; film technique" , "Being full of energy" , "Art of cartoon making" , "Cartoon technique" , "Making of cartoons" .)