Observer odds against to react badly (9)
I believe the answer is:
spectator
'observer' is the definition.
(spectator is a kind of observer)
'odds against to react badly' is the wordplay.
'odds' becomes 'sp' (betting abbreviation for starting price).
'against' is a charade indicator (letters next to each other).
'badly' is an anagram indicator.
'to'+'react'='toreact'
'toreact' with letters rearranged gives 'ectator'.
'sp'+'ectator'='SPECTATOR'
(Other definitions for spectator that I've seen before include "observer" , "member of the crowd" , "Cat poster (anag.)" , "one found at ground?" , "He attends matches" .)