Packs together to fix a bone (6)
I believe the answer is:
sacrum
'bone' is the definition.
(sacrum is a kind of bone)
'packs together to fix a' is the wordplay.
'packs together' becomes 'scrum' (I've seen this before).
'to fix' is an insertion indicator (inserted letters are fixed in place).
'scrum' going around 'a' is 'SACRUM'.
(Other definitions for sacrum that I've seen before include "Triangular bone forming back of pelvis" , "Triangular bone that forms back of pelvis" , "Wedge-shaped bone forming the back part of the pelvis" , "Bone in the lower back" .)