Leap — then react badly (9)
I believe the answer is:
entrechat
'leap' is the definition.
(type of jump in ballet)
'then react badly' is the wordplay.
'badly' is an anagram indicator.
'then'+'react'='thenreact'
'thenreact' is an anagram of 'ENTRECHAT'.
(Other definitions for entrechat that I've seen before include "Ballet movement" , "Ballet dancer's leap" , "Leap in the air" , "Ballet move" , "Vertical jump in ballet" .)