Question

What is the use of db_clean() with a simple example?

Answer 1

Seems like db_clean() is a customized helper which is calling xss_clean().

See this link, db_clean() is stored in MY_security_helper.php, whereby MY_ is the naming convention to extend native helper.

xss_clean()

Provides Cross Site Script Hack filtering. This function is an alias
to the one in the Input class. More info can be found there.

Source: https://www.codeigniter.com/user_guide/helpers/security_helper.html