Over period, modify highly informative device (9)
I believe the answer is:
altimeter
'highly informative device' is the definition.
'altimeter' can be an answer for 'device' (altimeter is a kind of device). I'm unsure of the 'highly informative' bit.
'over period modify' is the wordplay.
'over' indicates putting letters inside.
'period' becomes 'time period' (period of time).
'modify' becomes 'alter' (I've seen this in another clue).
'time' inserted within 'alter' is 'ALTIMETER'.
Can you help me to learn more?
(Other definitions for altimeter that I've seen before include "flying aid" , "Height sensor" , "Aircraft instrument" , "equipment for pilot" , "Instrument to measure height above ground" .)