Paul's inside after opening cake (6)
I believe the answer is:
gateau
'cake' is the definition.
(I know that gateau is a type of cake)
'paul's inside after opening' is the wordplay.
'inside' means to look at the middle letters.
'after' says to put letters next to each other.
'opening' becomes 'gate' (I have seen 'Movable opening ' mean 'gate' so perhaps 'opening' could also mean 'gate').
The middle letters of 'paul' are 'au'.
'au' put after 'gate' is 'GATEAU'.
(Other definitions for gateau that I've seen before include "rich food" , "named for Black Forest?" , "Rich cream cake" , "(French?) cake" , "Gooey cake" .)