Backbone and intestines (4)
I believe the answer is:
guts
I believe this is a double definition.
'backbone' is the first definition.
('guts' can be a synonym of 'backbone')
'intestines' is the second definition.
('gut' can be a synonym of 'intestine')
'and' is the link.
(Other definitions for guts that I've seen before include "Daring" , "Inside - bottle!" , "Courage; entrails" , "Intestines - courage" , "Fortitude and determination" .)