Chap beginning to imbibe cocktail (7)

I believe the answer is:

martini

'cocktail' is the definition.
(martini is a kind of cocktail)

'chap beginning to imbibe' is the wordplay.
'chap' becomes 'martin' (**).
'beginning to' suggests taking the first letters.
The initial letter of 'imbibe' is 'i'.
'martin'+'i'='MARTINI'

(Other definitions for martini that I've seen before include "alcoholic drink" , "tipple" , "Alternative name for Simone Memmi, pre-Renaissance Italian painter" , "Cocktail of gin and dry vermouth" , "rifle" .)

I've seen this clue in The Guardian.

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