Rates unusual zircon, initially, as imitation (6)
I believe the answer is:
ersatz
'imitation' is the definition.
(synonyms)
'rates unusual zircon initially' is the wordplay.
'unusual' indicates an anagram.
'initially' says to take the initial letters.
The initial letter of 'zircon' is 'z'.
'rates' is an anagram of 'ersat'.
'ersat'+'z'='ERSATZ'
'as' is the link.
(Other definitions for ersatz that I've seen before include "Inferior substitute" , "Spurious" , "'Artificial, not genuine (6)'" , "Made in imitation, from German" , "False" .)