Representation of one million years? (5)
I believe the answer is:
image
'representation' is the definition.
(image is a kind of representation)
'of one million years?' is the wordplay.
'of one' becomes 'i' (I've seen this in other clues).
'million' becomes 'm' (abbreviation).
'years?' becomes 'age' ('age' can be a synonym of 'years').
'i'+'m'+'age'='IMAGE'
(Other definitions for image that I've seen before include "Epitome" , "Likeness, might be spitting" , "Mental or physical representation" , "Visible representation" , "Representation of person or thing" .)