US anthem is fermenting feeling of excitement (10)
I believe the answer is:
enthusiasm
'feeling of excitement' is the definition.
(being excited or enthusiastic about something)
'us anthem is fermenting' is the wordplay.
'fermenting' indicates anagramming the letters.
'us'+'anthem'+'is'='usanthemis'
'usanthemis' with letters rearranged gives 'ENTHUSIASM'.
(Other definitions for enthusiasm that I've seen before include "Avidity" , "Ardour" , "Zeal" , "spirit?" , "Intense and lively interest" .)