Briefly perform without preparation, starting early to become better (7)
I believe the answer is:
improve
'become better' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for improve that I've seen before include "Become better" , "Change for the better" , "Polish" , "Get or make better" , "Train" .)