Reset custom variable to default value programmatically

Building up from the accepted answer I've made a function for it.

(defun custom/reset-var (symbl)
  "Reset SYMBL to its standard value."
  (set symbl (eval (car (get symbl 'standard-value)))))

(custom/reset-var 'somevar)

Another, probably cleaner, way to reset the value of a variable:

custom-reevaluate-setting is a compiled Lisp function in custom.el.

(custom-reevaluate-setting SYMBOL)

Reset the value of SYMBOL by re-evaluating its saved or standard value. Use the :set function to do so. This is useful for customizable options that are defined before their standard value can really be computed. E.g. dumped variables whose default depends on run-time information.

So for example:

(custom-reevaluate-setting 'emms-player-next-function)

will replace the previous value with that symbol's standard-value.

EDIT

Read @phils comment below for a very important detail regarding this function.

C-hig (elisp) Variable Definitions

Internally, defcustom uses the symbol property standard-value to record the expression for the standard value, saved-value to record the value saved by the user with the customization buffer, and customized-value to record the value set by the user with the customization buffer, but not saved. See Symbol Properties. These properties are lists, the car of which is an expression that evaluates to the value.

Hence:

(setq foo (eval (car (get 'foo 'standard-value))))

Note that this only applies to defcustom variables (i.e. "user options"). Emacs does not save the initial value of a defvar variable anywhere.