Fancy - game in one move (7)
I believe the answer is:
imagine
'fancy' is the definition.
(I know that fancy is a more specific form of imagine)
'game in one move' is the wordplay.
'one' becomes 'i' (Roman numeral).
'move' indicates anagramming the letters.
'game'+'in'+'i'='gameini'
'gameini' is an anagram of 'IMAGINE'.
(Other definitions for imagine that I've seen before include "John Lennon hit" , "See something that doesn't exist" , "Conjure up in mind,fancy" , "Conjure up in mind, visualise" , "John Lennon classic (just think of it!)" .)