Peculiar stone associated with mountains (7)
I believe the answer is:
strange
'peculiar' is the definition.
('strange' can be a synonym of 'peculiar')
'stone associated with mountains' is the wordplay.
'stone' becomes 'st'.
'associated with' means one lot of letters go next to another.
'mountains' becomes 'range' (I've seen this before).
'st'+'range'='STRANGE'
(Other definitions for strange that I've seen before include "as quarks in certain hadrons" , "Curious" , "Peculiar; Morse's boss" , "Odd - rum" , "Unknown or peculiar" .)