Drunk chose to drink lager at first (7)
I believe the answer is:
pickled
'drunk' is the definition.
(pickled can informally mean drunk)
'chose to drink lager at first' is the wordplay.
'chose' becomes 'picked' (picking is a kind of choosing).
'to drink' indicates putting letters inside.
'at first' suggests taking the first letters.
The initial letter of 'lager' is 'l'.
'picked' placed around 'l' is 'PICKLED'.
(Other definitions for pickled that I've seen before include "Blotto" , "Brahms and Liszt" , "Preserved in vinegar or brine" , "Drunk (as a gherkin?)" , "Preserved in liquid" .)