This is such a strange name, so I felt compelled to see where it comes from. The OED says ‘claper’ would be the Anglo-Norman version of the French ‘clapier’, which means rabbit hole. So a ‘clapper’ was a rabbit burrow, or maybe a place for deliberately keeping rabbits. The OED says for keeping ‘tame rabbits’ though I think they probably were often ‘tasty rabbits’. But not tasty for our prehistoric ancestors though, apparently rabbits only got established in the 13th century.
Hillforts were popular places for breeding Rabbits it would seem. I can think of another 2 off the top of my head with burrows. Though this is the only one with the name clappers.