Mark sent back drink of Burgundy which is sweet (6)

I believe the answer is:

gateau

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'is sweet' is the definition.
(type of dessert or sweet)

'mark sent back drink of burgundy' is the wordplay.
'mark' becomes 'tag' (synonyms).
'sent back' says the letters should be written backwards.
'drink of burgundy' becomes 'eau' (I can't explain this - if you can you should believe this answer much more).
'tag' reversed gives 'gat'.
'gat'+'eau'='GATEAU'

'which' is the link.

Can you help me to learn more?

(Other definitions for gateau that I've seen before include "named for Black Forest?" , "Cake for Pierre" , "Creamy dessert cake" , "Cake (from the Black Forest?)" , "Something French for 22?" .)

I've seen this clue in The Telegraph.

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