Travelling west in Navarre, I spot mountains (6)
I believe the answer is:
sierra
'mountains' is the definition.
(I've seen this before)
'travelling west in navarre i spot' is the wordplay.
'travelling west' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'arreis' is hidden within 'navarre i spot'.
'arreis' backwards is 'SIERRA'.
(Other definitions for sierra that I've seen before include "- - Leone" , "Mountain range -- code word for S" , "Comms code S" , "Long range" , "One range" .)