Lunar layout of a bone (5)
I believe the answer is:
ulnar
'of a bone' is the definition.
(relating to the ulna, an arm bone)
'lunar layout' is the wordplay.
'layout' is an anagram indicator (an alternative layout of the letters).
'lunar' with letters rearranged gives 'ULNAR'.
(Other definitions for ulnar that I've seen before include "Relating to one of the bones of the forearm" , "Relating to a particular arm bone" , "of certain bones" , "Relating to a bone in the forearm" .)