Bones in line, like back (5)
I believe the answer is:
sacra
'bones' is the definition.
(I've seen this before)
'line like back' is the wordplay.
'line' becomes 'arc' (arc is a kind of line**).
'like' becomes 'as'.
'back' says the letters should be written in reverse.
'arc'+'as'='arcas'
'arcas' reversed gives 'SACRA'.
'in' acts as a link.
(Other definitions for sacra that I've seen before include "Triangular bones" , "Bones that form the human pelvic arch" .)