The "k" in kg (4)
I believe the answer is:
kilo
'the k in kg' 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 "Metrical unit of weight" , "Lewis Carroll heroine" , "Metric unit of weight" , "Metric weight, informally" , "2.2 lbs" .)