The popular flet macro was deprecated in Emacs 24.3 and replaced with two similar macros - cl-flet and cl-letf.

flet was used to temporarily override function definitions. This was an analogue of a dynamically scoped let that operates on the function cell of symbols rather than their value cell.

The ability to dynamically rebind a functions was very useful for stubbing purposes in unit tests (you do write unit tests, don’t you?).

(flet ((projectile-project-root () "/path/to/project")
       (projectile-project-name () "project"))

projectile-project-root and projectile-project-name are impure functions (they depend on the current directory) and testing functions that use them internally would normally be problematic. However, flet gives us the ability to override their actual definitions in our tests. flet’s official replacement cl-flet is lexically bound and this is no longer possible with it.

Fortunately Nic Ferrier created a true drop-in flet replacement (with some extra magic baked in) - noflet. If you’re missing flet, I suggest you to give noflet a try.