Commando in mountains on run (6)
I believe the answer is:
ranger
'commando' is the definition.
(I know that commando can be written as ranger)
'mountains on run' is the wordplay.
'mountains' becomes 'range' (range of mountains).
'on' is a charade indicator (letters next to each other).
'run' becomes 'r' (cricket abbreviation).
'range'+'r'='RANGER'
'in' is the link.
(Other definitions for ranger that I've seen before include "Top guide" , "Forest or park keeper" , "Forest official" , "National park officer" , "Forest or park officer" .)