I am certainly shown to be better (8)
I believe the answer is:
improved
'be better' is the definition.
(similar in meaning)
'i am certainly shown' is the wordplay.
'am certainly' becomes 'm' (I can't justify this - if you can you should give a lot more credence to this answer).
'shown' becomes 'proved' (proving is a kind of showing).
'i'+'m'+'proved'='IMPROVED'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for improved that I've seen before include "Grew or made better" , "Became better" , "Made better in quality" , "Made or got better" , "Made progress" .)