What is your advice on writing "clean code" ?

Make it work first. Clean up later.

Perhaps 97 Things Every Programmer Should Know has some ideas.

FYI: "Always leave the campground cleaner than you found it." is The Boy Scout Rule.

• Base rules: a function should do only one thing, use descriptive variable names that make sense, document your code (explain WHY the code is there, not WHAT it does).
• Write your code so it can easily be expanded and refactored. Loosely coupled modules come to mind. You could use a boolean for a function parameter, but maybe a string/enum might be better if you foresee a binary parameter won't be enough in the near future.
• Don't try to be "smart" by overengineering the solution. This often leads to pointless abstractions for edge cases that may never happen. Premature optimization is a cardinal sin. "You Aren't Gonna Need It" (YAGNI)
• The bigger the project/feature you're building, the more upfront design you should do. If you intend to maintain the project for a very long time, spending some time on architecture design is worth it. If it is just a quick proof-of-concept kinda thing, why bother?
• An app that is never shipped does not exist. You can spend ages writing beautiful code, but your users won't care how the app looks on the inside. I don't mean you should rush writing code and take shortcuts all the time. At some point your code is just "good enough".

Sometimes you create a suboptimal implementation and decide to take on the technical debt (probably because of a deadline), while the software works as intended. If you don't intend on expanding the software, why would you touch the code again? Maybe you can leave it as is. Sometimes, a change in requirements can make the tech debt go away and the problem basically solved itself!

I'm not sure what clean code means.

And I'm pretty sure many understand it differently.

I prefer speaking about maintainability, scalability, readability, performance, security. Not every project need all of that, and not in the same proportions.

So, to answer the question: "it depends".

Always trying to leave code in better shape than how you found it is really a great idea.

You can't always have the entire big picture in mind, but having the mindset that you can continuously improve things locally as you go is a great way to ensure things stay clean.

If the goal is clean code, remember that there is no such thing as "later".

Make it work, and then commit that working code first so you have it, but then immediately follow that with a cleanup commit (so you can compare or revert if the cleanup breaks something).

Simple rule: Try not to break the "Single Responsibility Principle" (SRP), and most things will fall into place

Decide what "clean code" means to you. It's subjective

My advice to anyone who aspire to write maintainable software is to read the Twelve Factor App at least twice. It’s better the second time.

12factor.net/

I recommend "Good code, bad code" book.