Alfresco party is best (5)
I believe the answer is:
outdo
'is best' is the definition.
(I've seen this before)
'alfresco party' is the wordplay.
'alfresco' becomes 'out' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'party' becomes 'do' (do is a kind of party).
'out'+'do'='OUTDO'
Can you help me to learn more?
(Other definitions for outdo that I've seen before include "Exceed" , "Surpass someone at something" , "Perform better than surpass" , "Perform better than another" , "Beat" .)