Clock at one perhaps - have a drink (8)
I believe the answer is:
'a drink' is the definition.
(cocktail is a kind of drink)
'clock at one perhaps' is the wordplay.
'one' becomes 'i' (Roman numeral).
'perhaps' is an anagram indicator.
'clockati' with letters rearranged gives 'COCKTAIL'.
'have' is the link.
(Other definitions for cocktail that I've seen before include "Manhattan, for example" , "Screwdriver, for instance" , "old-fashioned in America" , "dish with prawn" , "Manhattan, perhaps" .)