Think of one play in game (7)
I believe the answer is:
imagine
'think of' is the definition.
(thesaurus)
'one play in game' is the wordplay.
'one' becomes 'i' (Roman numeral).
'play' is an anagram indicator.
'in'+'game'='ingame'
'ingame' anagrammed gives 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "Lennon song - dream" , "Suppose; think up" , "Assume" , "Conceive in the mind" , "Classic Lennon song" .)