Cocktail: a metric unit knocked back by girl (9)

I believe the answer is:

margarita

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'cocktail' is the definition.
(I know that Margarita is a type of cocktail)

'a metric unit knocked back by girl' is the wordplay.
'metric unit' becomes 'gram' (gram is a kind of metric unit).
'knocked back' says the letters should be written in reverse.
'by girl' becomes 'rita' (woman's name. I am not sure about the 'by' bit.).
'a'+'gram'='agram'
'agram' in reverse letter order is 'marga'.
'marga'+'rita'='MARGARITA'

Can you help me to learn more?

(Other definitions for margarita that I've seen before include "Stiff drink" , "Cocktail consisting of tequila and lemon juice" , "Type of tequila cocktail" , "Spanish girl" , "Tequila-based cocktail" .)

I've seen this clue in The Telegraph.

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