Unusual mountains covered with stone (7)
I believe the answer is:
strange
'unusual' is the definition.
(similar in meaning)
'mountains covered with stone' is the wordplay.
'mountains' becomes 'range' (range of mountains).
'covered with' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'stone' becomes 'st'.
'range' put after 'st' is 'STRANGE'.
(Other definitions for strange that I've seen before include "Extraordinary" , "Singular" , "- - Fruit (song)" , "Queer" , "Super for a couple here" .)