Clever Cleaner is an app produced by CleverFiles. The same people who make Disk Drill.
I have Disk Drill and it's a great file recovery app. But, I still wouldn't put Clever Cleaner on any of my devices, even being free.
As IdrisSeabright noted, it doesn't do anything that isn't already built-in to iOS, or you couldn't otherwise do yourself. This is true of any cleaning software (in particular). iOS is a sandboxed OS, meaning apps can't touch the system, or any other app. They can, however, do almost anything they want to your personal data (photos, cookies, contacts, etc.).
The problem with cleaners is they have to guess what's junk and what isn't. They may, and do, remove things that are important to you. But they don't know that, and that's the entire problem with them.
In short, you should never hand over control of your data to a third party app. If you use one that only points out things like duplicates, but takes no further action, that's fine. But it should always be up to you to make the final, manual decision to decide what you do or don't need.