Spontaneous (11)
'spontaneous' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for impromptu that I've seen before include "Unplanned; composition" , "Without rehearsal or preparation" , "Unprepared" , "Extemporaneous speech" , "On the spur of the moment" .)
'spontaneous' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for automatic that I've seen before include "lacking personal control" , "Like George" , "Self-loading" , "Type of gun or car" , "Unmanned" .)
'spontaneous' is the definition.
(similar in meaning)
This is the entire clue.
(Other definitions for impulsive that I've seen before include "Likely to act without considering the consequences" , "Spontaneous" , "Done without forethought" , "Hot-headed" , "responding promptly" .)
'spontaneous' is the definition.
(I know that spontaneous can be written as ad-lib)
This is the entire clue.
(Other definitions for ad-lib that I've seen before include "Speak off the cuff" , "Spontaneously" , "Talk without notes" , "without restraint" , "Depart from the text" .)
'spontaneous' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for unrehearsed that I've seen before include "Sixteen across [OFF THE TOP OF ONES HEAD]" , "Off-the-cuff" , "Not practised in advance" , "(Of play) not prepared" , "Not practised beforehand" .)
(Other definitions for instant that I've seen before include "type of coffee" , "Tick" , "The blink of an eye" , "Second - point in time" , "Short time; immediate" .)