Rugby formation picking up a bone (6)
I believe the answer is:
sacrum
'bone' is the definition.
(I know that sacrum is a type of bone)
'rugby formation picking up a' is the wordplay.
'rugby formation' becomes 'scrum' (I've seen this in another clue).
'picking up' means one lot of letters goes inside another.
'scrum' placed around 'a' is 'SACRUM'.
(Other definitions for sacrum that I've seen before include "Triangular bone forming back of pelvis" , "Wedge-shaped bone forming the back part of the pelvis" , "Bone in the lower back" , "Triangular bone that forms back of pelvis" .)