Unusual arrangement of garnets (7)
I believe the answer is:
strange
'unusual' is the definition.
('strange' can be similar in meaning to 'unusual')
'arrangement of garnets' is the wordplay.
'arrangement of' indicates an anagram.
'garnets' with letters rearranged gives 'STRANGE'.
(Other definitions for strange that I've seen before include "Odd or unfamiliar" , "Offbeat" , "Odd, peculiar" , "Odd, alien" , "Peculiar; Morse's boss" .)