find-tag-default-function

find-tag-default-function is a variable defined in `etags.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
A function of no arguments used by M-x find-tag to pick a default tag.
If nil, and the symbol that is the value of `major-mode'
has a `find-tag-default-function' property (see `put'), that is used.
Otherwise, `find-tag-default' is used.

You can customize this variable.