Measurement from top to bottom (6)
I believe the answer is:
height
'measurement from top to bottom' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for height that I've seen before include "Measurement from top to bottom" , "Vertical extent" , "Dimension from top to bottom" , "Distance above ground or sea" , "Rising ground; extreme example" .)