Make certain drink by bar (6-5)

I believe the answer is:

double-check

'make certain' is the definition.
(double-checking is a kind of making certain)

'drink by bar' is the wordplay.
'drink by' becomes 'double' (double shot of alcohol. I am not sure about the 'by' bit.).
'bar' becomes 'check' (check can mean to bar or control).
'double'+'check'='DOUBLE-CHECK'

I've seen this clue in The Guardian.

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