Do well to answer verbally. "What is XX plus XX?" (5)
I believe the answer is:
'do well' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for excel that I've seen before include "Do really well, outshine others", "cap", "Be outstandingly proficient", "Outrival", "well above average".)