After barrier, gold cake! (6)
I believe the answer is:
gateau
'cake' is the definition.
(I know that gateau is a type of cake)
'after barrier gold' is the wordplay.
'after' says to put letters next to each other.
'barrier' becomes 'gate' (gate is a kind of barrier).
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'gate' next to 'au' is 'GATEAU'.
(Other definitions for gateau that I've seen before include "Fancy cake" , "Cake for Pierre" , "Richly decorated cake" , "named for Black Forest?" , "Black Forest dessert" .)