cl-letf is an autoloaded Lisp macro in `cl-macs.el'.

(cl-letf ((PLACE VALUE) ...) BODY...)

Temporarily bind to PLACEs.
This is the analogue of `let', but with generalized variables (in the
sense of `setf') for the PLACEs. Each PLACE is set to the corresponding
VALUE, then the BODY forms are executed. On exit, either normally or
because of a `throw' or error, the PLACEs are set back to their original
values. Note that this macro is *not* available in Common Lisp.
As a special case, if `(PLACE)' is used instead of `(PLACE VALUE)',
the PLACE is not modified before executing BODY.