Odd mountains, by the way (7)
I believe the answer is:
strange
'odd' is the definition.
(thesaurus)
'mountains by the way' is the wordplay.
'mountains' becomes 'range' (I've seen this before).
'by' says to put letters next to each other.
'the way' becomes 'st'.
'range' put after 'st' is 'STRANGE'.
(Other definitions for strange that I've seen before include "- - Fruit (song)" , "Odd, alien" , "Odd, not familiar" , "Foreign" , "Offbeat" .)