Height gauge (9)
I believe the answer is:
altimeter
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">height gauge</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for altimeter that I've seen before include "Plane's instrument" , "Instrument for measuring height" , "Plane's height measurer" , "flying aid" , "Instrument that measures the height above ground" .)