Bone somewhat twisted, restricting one (5)
I believe the answer is:
tibia
'bone' is the definition.
(tibia is a kind of bone)
'somewhat twisted restricting one' is the wordplay.
'somewhat' becomes 'abit' (a bit).
'twisted' is a reversal indicator.
'restricting' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'abit' in reverse letter order is 'tiba'.
'tiba' placed around 'i' is 'TIBIA'.
(Other definitions for tibia that I've seen before include "part of skeleton" , "Shin bone" , "Part of leg of" , "Bone in osso bucco" , "The large shinbone" .)