Surpass alfresco celebration (5)
I believe the answer is:
outdo
'surpass' is the definition.
(I know that surpass can be written as outdo)
'alfresco celebration' is the wordplay.
'alfresco' becomes 'out' (an alfresco meal is eaten outside).
'celebration' becomes 'do' (I've seen this in other clues).
'out'+'do'='OUTDO'
(Other definitions for outdo that I've seen before include "going to top?" , "Surpass - overshadow - excel" , "Overcome" , "Cap" , "Perform better than (someone)" .)