The “a-” is an unstressed form of “of” and the word has been a regular member of the English lexicon since the 1200s.

• 17.048 and howls rent the air afar.
• 18.021 and at length smelling battle from afar
• 19.031 Turn at last to home afar.

“afar, adv.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.