Pastry case, but no filling portion (5)
I believe the answer is:
piece
'portion' is the definition.
(I know that piece is a type of portion)
'pastry case but no filling' is the wordplay.
'pastry' becomes 'pie' (pie is a kind of pastry).
'but no filling' suggests removing the centre.
'case' with its middle taken out is 'ce'.
'pie'+'ce'='PIECE'
(Other definitions for piece that I've seen before include "Separate part of a whole" , "Section; firearm" , "Castle, perhaps" , "Segment" , "One part of a whole" .)