You're seeing just the macro def_clock, go back to Membrane.Element.Base module for more information.
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def_clock(doc \\ "")

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Defines that element exports a clock to pipeline.

Exporting clock allows pipeline to choose it as the pipeline clock, enabling other elements to synchronize with it. Element's clock is accessible via clock field, while pipeline's one - via parent_clock field in callback contexts. Both of them can be used for starting timers.