How drink might be downed at home by individual (2,3)
I believe the answer is:
in one
'how drink might be downed' is the definition.
(a drink may be downed in one gulp)
'home by individual' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'by' means one lot of letters go next to another.
'individual' becomes 'one' (an individual is one person).
'in'+'one'='IN ONE'
'at' is the link.
(Other definitions for in one that I've seen before include "Combined" , "united" , "A hole . . . . . is a singular golf score" , "As a single unit" .)