dnd 5e – How do you cast Mending on a clockwork character?

Per the rules you linked, by default, you’re a construct, and therefore, per the rules for most (all?) of the healing spells provided so far, you can’t be magically healed. Mending only stabilizes, it doesn’t heal, so that doesn’t help.

Seems like the only way to be magically healed is if you selected the Alchemical Blood trait as one of your Maker’s Marks:

Alchemical Blood: You have an alchemical mixture that keeps you running. Despite your construct nature, you are considered humanoid when a creature casts a spell to grant you hit points.

In practice, being healable is basically necessary to function in most D&D campaigns, so almost all playable Clockworks would take Alchemical Blood. Which sounds like a limitation, but most of the other Maker’s Marks are so bad/useless that you’re not missing out on much.

I’ll second the suggestion from others that you avoid homebrew for your first campaign. If you want to play a living construct, play a Warforged, which isn’t nearly so complicated/poorly thought out.