A long time making one change (3)
I believe the answer is:
eon
'a long time' is the definition.
(I've seen this before)
'one change' is the wordplay.
'change' indicates an anagram.
'one' anagrammed gives 'EON'.
'making' acts as a link.
(Other definitions for eon that I've seen before include "Geological period" , "Era" , "An immense time" , "Very long age" , "Immense period" .)