Excel with an alfresco celebration (5)
I believe the answer is:
outdo
'excel' is the definition.
(I've seen this before)
'with an alfresco celebration' is the wordplay.
'with' means one lot of letters go next to another.
'an alfresco' becomes 'out' (an alfresco meal is eaten outside).
'celebration' becomes 'do' (I've seen this in other clues).
'out' put next to 'do' is 'OUTDO'.
(Other definitions for outdo that I've seen before include "going to top?" , "Perform better than, be superior to" , "Cap" , "Surpass - overshadow - excel" , "Overcome" .)