Best advertisement for cake (5,4)
I believe the answer is:
cream puff
'cake' is the definition.
(I have seen 'Rich cake ' mean 'cream puff' so perhaps 'cake' could also mean 'cream puff')
'best advertisement' is the wordplay.
'best' becomes 'cream' (as in 'the cream of the crop').
'advertisement' becomes 'puff' (I've seen this before).
'cream'+'puff'='CREAM PUFF'
'for' acts as a link.
(Other definitions for cream puff that I've seen before include "Rich cake" , "Pastry confection" .)