Work beginning for office: drink up! (4)
I believe the answer is:
opus
'work' is the definition.
(an opus is an artistic work)
'beginning for office drink up' is the wordplay.
'beginning for' says to take the initial letters.
'drink' becomes 'sup' (to sup is to drink).
'up' is a reversal indicator (in down clue: letters go upwards).
The initial letter of 'office' is 'o'.
'sup' backwards is 'pus'.
'o'+'pus'='OPUS'
(Other definitions for opus that I've seen before include "Artistic work" , "Composer's catalogue number" , "Work or composition, usually musical" , "Work (of a composer?)" , "Work of 25 perhaps" .)