Biting a pastry (4)
I believe the answer is:
tart
I believe this is a double definition.
'biting' is the first definition.
(thesaurus)
'a pastry' is the second definition.
(tart is a kind of pastry)
(Other definitions for tart that I've seen before include "Bitter - sweet - floozie" , "Acidulous" , "Sharp or sour in taste" , "Bitter - pudding" , "Sharp, acidic" .)