Lizard appears on the screen (7)

I believe the answer is:

monitor

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'lizard' is the definition.
(monitor lizard is a kind of lizard)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for monitor that I've seen before include "Keep a check on" , "Keep an eye on" , "Person or device for observing and checking" , "Supervise" , "record" .)

I've seen this clue in the Sydney Morning Herald.

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