Dot on a monitor (5)
I believe the answer is:
pixel
'dot on a monitor' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pixel that I've seen before include "Tiny point on display" , "Coloured dot in a screen picture or image" , "Dot on a TV screen" , "content of screen" , "Point on a screen" .)