Classify mountains (5)
I believe the answer is:
range
I believe this clue is a double definition.
'classify' is the first definition.
(range can mean to rank or classify)
'mountains' is the second definition.
(range of mountains)
(Other definitions for range that I've seen before include "Distance between weapon and target" , "sight" , "A set of different things of the same general type" , "specific category" , "Cut; cultivated plant" .)