Put off in high-altitude terrain (5)
I believe the answer is:
deter
'put off' is the definition.
(both can mean to discourage)
'in high-altitude terrain' is the wordplay.
'in' indicates the answer is hidden within the clue.
'DETER' can be found hidden inside 'high altitude terrain'.
(Other definitions for deter that I've seen before include "Warn off" , "Inhibit" , "Try to prevent" , "Cause not to do something" , "Check" .)