How great age changes one! (3)
I believe the answer is:
eon
'how great age' is the definition.
'eon' can be an answer for 'age' (eon is a kind of age). I'm not sure about the 'how great' bit.
'changes one' is the wordplay.
'changes' indicates anagramming the letters.
'one' anagrammed gives 'EON'.
Can you help me to learn more?
(Other definitions for eon that I've seen before include "Age, epoch" , "Long period of time" , "Very long age" , "In time, long division" , "Vast age (alt. sp.)" .)