Terrible pains to the left of the backbone (6)
I believe the answer is:
spinal
'of the backbone' is the definition.
(I've seen this before)
'terrible pains to the left' is the wordplay.
'terrible' indicates anagramming the letters.
'to the left' becomes 'L' (common abbreviation).
'pains' with letters rearranged gives 'spina'.
'spina'+'l'='SPINAL'
(Other definitions for spinal that I've seen before include "like a column in body" , "of a vital column" , "Relating to the backbone" , "Type of column" , "Plains (anag.)" .)