Meal taken by most of old France constitutes cake (6)
I believe the answer is:
gateau
'cake' is the definition.
(gateau is a kind of cake)
'meal taken by most of old france' is the wordplay.
'meal' becomes 'tea' (tea is a type of meal).
'taken by' indicates putting letters inside.
'most of' means to remove the last letter.
'old france' becomes 'gaul' (I've seen this in another clue).
'gaul' with its last letter taken away is 'gau'.
'tea' inserted into 'gau' is 'GATEAU'.
'constitutes' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for gateau that I've seen before include "It's sweet" , "Fancy cake" , "Rich dessert cake" , "Something French for 22?" , "Creamy dessert cake" .)