As explained by @Charlie. you can use a Boolean variable to track if the Reader has seen the clock or not.
1. Initialise your story variable in you're project's startup tagged special passage.
If your project doesn't have such a special passage yet then first create a new Passage (the Name isn't important and can be anything you like, I name mine Startup) then assign it a Passage Tag of startup in all lower-case letters.
Add the following to this passage, it defaults the story variable to false.
(set: $SeenClock to false)
2. Change story variable when the Reader sees the clock.
Add code like the following to the Look at the clock passage meantioned in @Charlie's example.
(set: $SeenClock to true)
3. Use the current value of the story variable to control the displaying of the link.
Use code like the following in the Passage you want the Look at the clock link to conditionally appear in.
(if: not $SeenClock)[
[[Look at the clock]]