Pillar, backbone (8)
I believe the answer is:
mainstay
I believe this clue is a double definition.
'pillar' is the first definition.
(I know this)
'backbone' is the second definition.
(I know this)
(Other definitions for mainstay that I've seen before include "Pillar of strength" , "big support" , "Chief source of support" , "Linchpin, principle support" , "Rope to the mast is the chief support" .)