Cabbage and pastry? That's not unknown (4)
I believe the answer is:
chou
'cabbage' is the definition.
(I know that cabbage can be written as chou)
'pastry? that's not unknown' is the wordplay.
'pastry?' becomes 'choux' (I've seen this before).
'that's not' suggests deleting specific letters (that has not).
'unknown' becomes 'x' (algebra).
'choux' with 'x' removed is 'CHOU'.
'and' is the link.
(Other definitions for chou that I've seen before include "Cabbage - Chinese imperial dynasty" , "Pet" , "Cabbage - ornamental rosette - cream bun" , "Cabbage-shaped pastry" .)