Long pastry must be filled with one in the middle (4)
I believe the answer is:
pine
'long' is the definition.
('pine' can be a synonym of 'long')
'pastry must be filled with one in the middle' is the wordplay.
'pastry' becomes 'pie' (pie is a kind of pastry).
'must be filled with' indicates putting letters inside.
'in the middle' says to take the centre.
The central letter of 'one' is 'n'.
'pie' going around 'n' is 'PINE'.
(Other definitions for pine that I've seen before include "Kind of needle" , "Conifer grown for wood" , "type of wood" , "Scots fir" , "Long - timber" .)