Aircraft instrument (9)
I believe the answer is:
altimeter
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">aircraft instrument</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for altimeter that I've seen before include "Plane's instrument" , "Aircraft instrument" , "Instrument for measuring height" , "equipment for pilot" , "It measures height above sea level" .)