Crossword Tracker states that the most likely answer to the crossword clue "radius neighbor" is ulna. In the clue, radius refers to the bone in the forearm, which lays parallel to the ulna.
The use of the clue "radius neighbor" traces back to at least 1982, when it was used in a crossword puzzle published by The New York Times, according to Crossword Tracker. The newspaper used the clue in other puzzles as well, including the one from March 26, 2014. The LA Times, The Wall Street Journal, Washington Post and USA Today have also used the clue "radius neighbor" in earlier publications.