Change round hours and minutes perhaps in measuring instrument (9)
I believe the answer is:
altimeter
'measuring instrument' is the definition.
(altimeter is a kind of measuring instrument)
'change round hours and minutes perhaps' is the wordplay.
'change' becomes 'alter' ('alter' can be a synonym of 'change').
'round' is an insertion indicator.
'hours and minutes perhaps' becomes 'time'.
'alter' going around 'time' is 'ALTIMETER'.
'in' acts as a link.
(Other definitions for altimeter that I've seen before include "Aircraft height indicator" , "Instrument for measuring height" , "Plane's instrument" , "It measures height above sea level" , "Flyer's instrument" .)