Excitedly anticipating a modest backing (8)

I believe the answer is:

aflutter

'excitedly anticipating' is the definition.

'a modest backing' is the wordplay.
'modest backing' becomes 'flutter' (flutter can mean a small bet).
'a'+'flutter'='AFLUTTER'

(Other definitions for aflutter that I've seen before include "trembling" , "Agitated - excited (eg in anticipation)" , "is rather nervous" , "Nervous, twitchy" , "in excited state" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.