Pastry's sour: bit of tarragon the French included (7)
I believe the answer is:
tartlet
'pastry's' is the definition.
(tartlet is a kind of pastry)
'sour bit of tarragon the french included' is the wordplay.
'sour' becomes 'tart' (similar in meaning).
'bit of' says to take the initial letters.
'the french' becomes 'le' ('the' in French).
'included' is an insertion indicator.
The initial letter of 'tarragon' is 't'.
'tart'+'t'='tartt'
'tartt' placed around 'le' is 'TARTLET'.
(Other definitions for tartlet that I've seen before include "Small quiche" , "little cake" , "single pastry" , "Small open pastry" , "Food" .)