Be superior (5)
'be superior' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for excel that I've seen before include "Be highly proficient" , "Outrival" , "cap" , "Eclipse" , "Be exceptionally good (at something)" .)
(Other definitions for prevail that I've seen before include "Succeed ultimately" , "Be widespread; prove superior" , "get the upper hand" , "Triumph, overcome" , "Have opinion accepted" .)