Measuring device used in extreme terrain (5)
I believe the answer is:
meter
'measuring device' is the definition.
(I've seen this before)
'in extreme terrain' is the wordplay.
'in' says the answer is hidden in the clue.
'METER' is hidden in the letters of 'extreme terrain'.
'used' is the link.
(Other definitions for meter that I've seen before include "Apparatus for measuring" , "Instrument for measuring quantity" , "Something measuring" , "Device for recording, say, usage (5)" , "Measuring device as for electricity consumption" .)