Knock sharply (3)
I believe the answer is:
rap
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">knock sharply</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for rap that I've seen before include "black music is a > blow" , "Form of music with chanted rhyming lyrics" , "Rhyming monologue recited to rock music below par" , "Strike; music genre" , "Sharp blow; criticism" .)