An arm or a leg (4)
I believe the answer is:
limb
I believe this is a double definition.
'an arm' is the first definition.
(I know that arm can be written as limb)
'a leg' is the second definition.
(I've seen this before)
(Other definitions for limb that I've seen before include "Human's extension" , "Arm, leg or wing" , "as part of plane, say" , "Eg wing" , "Bough" .)