Do really well (5)
I believe the answer is:
excel
'do really well' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for excel that I've seen before include "Be exceptionally good (at a subject)" , "Perform exceptionally well" , "well above average" , "Be exceptionally good (at something)" , "surpass all others" .)