Best garden party? (5)
I believe the answer is:
outdo
'best' is the definition.
(I know that best can be written as outdo)
'garden party?' is the wordplay.
'garden' 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'
(Other definitions for outdo that I've seen before include "Get the better of" , "Perform better than, be superior to" , "Exceed" , "Be better in performance (than another person)" , "Beat" .)