A good journalist up in years (4)
I believe the answer is:
aged
'up in years' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a good journalist' is the wordplay.
'good' becomes 'g' (abbreviation).
'journalist' becomes 'ed' (short for editor).
'a'+'g'+'ed'='AGED'
Can you help me to learn more?
(Other definitions for aged that I've seen before include "Mature" , "Long in the tooth" , "not a modern copy" , "Old or ancient" , "well over the hill" .)