Focus on time passing:
* The clock ticked like a heartbeat, reminding me of the fleeting nature of time.
* The clock hands moved as slow as molasses in January, making the wait feel unbearable.
* The clock was like a relentless drumbeat, counting down the seconds until the deadline.
* Time flew by like a runaway train, leaving the clock behind.
Focus on the clock's appearance:
* The grandfather clock stood tall and imposing, like a silent sentinel in the corner.
* The clock face was as blank and unreadable as a frozen lake.
* The clock's pendulum swung back and forth like a metronome, keeping a steady rhythm.
Focus on the clock's sound:
* The clock's chime was as sweet as a lullaby, soothing me to sleep.
* The clock's ticking was like a whisper, barely audible in the stillness.
* The clock's alarm sounded like a siren, jolting me awake.
The best simile will depend on the context and the specific aspect of the clock you want to highlight.