Start working after drinking a short (4)
I believe the answer is:
open
'start' is the definition.
(opening is a kind of starting)
'working after drinking a short' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'after drinking' indicates putting letters inside.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'short' means to remove the last letter.
'per' with its last letter taken away is 'pe'.
'on' placed around 'pe' is 'OPEN'.
(Other definitions for open that I've seen before include "Not shut or secret" , "With which to offer hearty welcome" , "Exposed to the air" , "ready to entertain new ideas" , "Undecided" .)