Change round hour and position indicator (9)
I believe the answer is:
altimeter
'position indicator' is the definition.
'change round hour' is the wordplay.
'change' becomes 'alter' ('alter' can be a synonym of 'change').
'round' indicates putting letters inside.
'hour' becomes 'time'.
'alter' enclosing 'time' is 'ALTIMETER'.
'and' is the link.
(Other definitions for altimeter that I've seen before include "Instrument for measuring height" , "flying aid" , "Aircraft instrument" , "Height sensor" , "Plane's instrument" .)