It is indeed a ½ Anna stamp from Nepal. It looks genuine, but it is a very common stamp and has minimal value. The cancellation is part of the large and ornate telegraph cancellation. These are much more common on these ½ Anna stamps than actual postal cancellations. You could once (and maybe still can) buy whole sheets of the ½ Anna with telegraph cancels.
These are typical
postal cancellations of that period:
