Excitedly anticipating a modest backing (8)
I believe the answer is:
Here is my best explanation:
'excitedly anticipating' is the definition.
'a modest backing' is the wordplay.
'modest backing' becomes 'flutter' (flutter can mean a small bet).
(Other definitions for aflutter that I've seen before include "trembling", "is rather nervous", "Agitated - excited (eg in anticipation)", "> agitated", "Nervous, twitchy" and "in excited state".)