Think a Gemini is fantastic (7)
I believe the answer is:
imagine
'think' is the definition.
(I know that think is a more specific form of imagine)
'a gemini is fantastic' is the wordplay.
'is fantastic' indicates an anagram (a fantastic or improbable ordering of the letters).
'a'+'gemini'='agemini'
'agemini' with letters rearranged gives 'IMAGINE'.
(Other definitions for imagine that I've seen before include "Enigma, I? (anag)" , "John Lennon classic (just think of it!)" , "Form a mental image in the mind" , "Fancy - dream" , "Suppose; think up" .)