In nervous state, having a modest bet (8)
I believe the answer is:
aflutter
'in nervous state' is the definition.
(aflutter can mean in an excited or nervous state)
'a modest bet' is the wordplay.
'modest bet' becomes 'flutter' (flutter can mean a small bet).
'a'+'flutter'='AFLUTTER'
'having' is the link.
(Other definitions for aflutter that I've seen before include "Nervous, twitchy" , "Agitated - excited (eg in anticipation)" , "in excited state" , "trembling" , "is rather nervous" .)