Curious stone associated with mountains (7)
I believe the answer is:
strange
'curious' is the definition.
(synonyms)
'stone associated with mountains' is the wordplay.
'stone' becomes 'st' (abbreviation).
'associated with' says to put letters next to each other.
'mountains' becomes 'range' (range of mountains).
'st'+'range'='STRANGE'
(Other definitions for strange that I've seen before include "Odd, peculiar" , "- - Fruit (song)" , "Out of the ordinary" , "Bizarre" , "Singular" .)