Do better than extra large, say (5)
I believe the answer is:
excel
'do better than' is the definition.
(I've seen this before)
'extra large say' is the wordplay.
'extra large' becomes 'XL' (abbreviation).
'say' shows a homophone (sound like).
'xl' is a homophone of 'EXCEL'.
(Other definitions for excel that I've seen before include "Every one" , "Outdo" , "Outrival" , "Be exceptionally good (at a subject)" , "cap" .)