Change outside of one meteorological flight instrument (9)
I believe the answer is:
altimeter
'instrument' is the definition.
(altimeter is a kind of instrument)
'change outside of one meteorological flight' is the wordplay.
'change' becomes 'alter' (synonyms).
'outside' is an insertion indicator.
'of one' becomes 'i' (I've seen this in other clues).
'meteorological flight' becomes 'met' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'met'='imet'
'alter' going around 'imet' is 'ALTIMETER'.
Can you help me to learn more?
(Other definitions for altimeter that I've seen before include "Plane's height measurer" , "flying aid" , "It measures height above sea level" , "Aircraft height indicator" , "Instrument that measures the height above ground" .)