Ad-libbing (13)
I believe the answer is:
improvisation
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ad-libbing</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for improvisation that I've seen before include "Extempore performance" , "Spontaneous performance" , "unrehearsed piece" , "Ad lib performance" , "Ad-libbing" .)