Anthem and poem are thus improved (7)
I believe the answer is:
emended
'improved' is the definition.
(emending is a kind of improving)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for emended that I've seen before include "Rectified" , "Improved" , "subjected to correction" , "Edited" , "Made changes to" .)