how to clear disk space ono a mac that doesn't start up

Startup the Mac up in Recovery Mode,

as per > About macOS Recovery on Intel-based Mac computers - Apple Support

Vumedialab Said:

"how to clear disk space ono a mac that doesn't start up"

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Troubleshooting Power Issues:

I. Hardware Use:

First off, go brand-for-brand (Apple-for-Apple) is best when it comes to reliability and consistency between devices (based on personal experience). So, if third-party, the a hardware may have damaged the Mac.

II. Troubleshooting Power:

A. Try a New Charger:

• Power & Cables - Mac Accessories - Apple - Select "Mac Compatibility" on the left
• Find the right power adapter and cable for your Mac notebook - Apple Support

B. A Few Links to Refer to for Power:

• If your Mac doesn't turn on - Apple Support

• How to turn your Mac on or off - Apple Support

• About Mac notebook batteries - Apple Support

• Charge the MacBook Pro battery - Apple Suppor

• If your Mac battery won't charge - Apple Support

You cannot really. The real question is why won' t it start up?

• No power?
• Has the drive crashed??
• Some other issue???

Please define "doesn't start up".