Bones bend when set the wrong way (5)
I believe the answer is:
sacra
'bones' is the definition.
(sacrum is a kind of bone)
'bend when set the wrong way' is the wordplay.
'bend' becomes 'arc' (arcing is a kind of bending).
'when set' becomes 'as' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the wrong way' shows that the letters should be reversed in order.
'arc'+'as'='arcas'
'arcas' in reverse letter order is 'SACRA'.
Can you help me to learn more?
(Other definitions for sacra that I've seen before include "Triangular bones" , "Bones that form the human pelvic arch" .)