Bureau assistant's last work computer (7)
I believe the answer is:
desktop
'computer' is the definition.
(desktop computer is a kind of computer)
'bureau assistant's last work' is the wordplay.
'bureau' becomes 'desk' (I've seen this before**).
'last' suggests the final letters.
'work' becomes 'op' (abbreviation for opus).
The last letter of 'assistant' is 't'.
'desk'+'t'+'op'='DESKTOP'
(Other definitions for desktop that I've seen before include "Work surface; computer type" , "board?" , "working area in office" , "Working surface, computer screen area" , "Background in computing" .)