Unit of frequency (5)
I believe the answer is:
hertz
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unit of frequency</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for hertz that I've seen before include "Cycles per second unit" , "A measure of frequency" , "Unit of frequency - German physicist, d. 1894" .)