I'm in a state of excitement (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' (synonyms).
'a'+'nation'='anation'
'im' going into 'anation' is 'ANIMATION'.
'of' is the link.
(Other definitions for animation that I've seen before include "Art of cartoon making" , "Liveliness and vigour" , "Liveliness - illusion of movement in cartoon films" , "Cartoon-making art" , "Making of cartoons" .)