One has the makings of a great time (3)
I believe the answer is:
eon
'a great time' is the definition.
(I've seen this in another clue)
'one has the makings of' is the wordplay.
'has the makings of' indicates an anagram (the anagram can be used to make the solution).
'one' anagrammed gives 'EON'.
(Other definitions for eon that I've seen before include "Major division of geological time" , "billion years" , "Geological period" , "In time, long division" , "thousand million years" .)