Transient, changing immediately (9)
I believe the answer is:
instanter
'immediately' is the definition.
(thesaurus)
'transient changing' is the wordplay.
'changing' indicates anagramming the letters.
'transient' with letters rearranged gives 'INSTANTER'.
(Other definitions for instanter that I've seen before include "without delay" , "On the spot" , "in court, immediately!" , "without hesitation" .)