Fake member of parliament initially says Latin must be included (3,3)
'fake member' is the definition.
(member can mean a limb)
I don't understand the rest of the clue.
(Other definitions for peg leg that I've seen before include "Informally, an artificial lower limb", "Eg. Long John Silver", "(Person with) artificial limb", "Artificial limb (usually of wood)", "Long John Silver, for instance".)