The "k" of kHz (4)
I believe the answer is:
kilo
'the k of khz' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for kilo that I've seen before include "Metric unit of weight" , "Unit of weight (abbr)" , "Unit of mass" , "2.2 lbs" , "more than two pounds" .)