Speaking without preparation (2-7)
'speaking without preparation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ad-libbing that I've seen before include "Speaking without preparation" , "Improvising" .)
(Other definitions for improvisation that I've seen before include "Ad-libbing" , "Extempore performance" , "Ad-lib" , "Ad lib performance" , "unrehearsed piece" .)