Fancy coming unstuck in one game (7)
I believe the answer is:
imagine
'fancy' is the definition.
(I know that fancy is a kind of imagine)
'unstuck in one game' is the wordplay.
'unstuck' indicates anagramming the letters (letters come unstuck from their original positions).
'one' becomes 'i' (Roman numeral).
'in'+'i'+'game'='inigame'
'inigame' anagrammed gives 'IMAGINE'.
'coming' is the link.
(Other definitions for imagine that I've seen before include "Suppose; think up" , "Reckon" , "Fancy - Lennon song" , "Picture mentally" , "Enigma, I? (anag)" .)