Stone cooking system is unusual (7)
I believe the answer is:
strange
'unusual' is the definition.
(I know that unusual can be written as strange)
'stone cooking system' is the wordplay.
'stone' becomes 'st' (abbreviation).
'cooking system' becomes 'range' (I can't explain this - if you can you should believe this answer much more).
'st'+'range'='STRANGE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for strange that I've seen before include "Unusual, odd" , "Peculiar; Morse's boss" , "Eccentric" , "- - Fruit (song)" , "Odd or unknown" .)