Bones in line while erect (5)
I believe the answer is:
sacra
'bones' is the definition.
(sacrum is a kind of bone)
'line while erect' is the wordplay.
'line' becomes 'arc' (arc is a kind of line).
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'erect' says the letters should be written in reverse.
'arc'+'as'='arcas'
'arcas' in reverse letter order is 'SACRA'.
'in' is the link.
(Other definitions for sacra that I've seen before include "Triangular bones" , "Bones that form the human pelvic arch" .)