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