Sort of pastry half filled with duck (4)
I believe the answer is:
filo
'sort of pastry' is the definition.
(I know that filo is a type of puff pastry)
'half filled with duck' is the wordplay.
'half' means to take half the letters.
'with' means one lot of letters go next to another.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'filled' cut in half is 'fil'.
'fil'+'o'='FILO'
(Other definitions for filo that I've seen before include "Thin flaky pastry" , "Kind of pastry" , "Tissue-thin sheets of pastry" , "A type of very thin pastry" , "Pastry made in thin sheets" .)