Computer screen (3)
(Another definition for vdu that I've seen is " Visual display unit (abbrev.)".)
'computer screen' is the definition.
(I know that computer screen can be written as monitor)
This is all the clue.
(Other definitions for monitor that I've seen before include "Observe over time" , "Watch - lizard - warship - VDU" , "Keep an eye on" , "Vet" , "Keep a watching brief" .)