Who is Lord Chandikeswara in Lord Shiva's temples?
Whats the purpose behind clapping in front of him?
Lord Chandikeshvara is the person who maintains a record of the devotees - sort of like Chitragupta. Actually the real practice is not clapping in front of his Sannidhi. You have to rub your hands clean and pray to him. This symbolises that we are taking nothing but Lord Shiva's grace from the temple.
Reference: Smt Desa Mangayarkarasi's discourse (the YouTube link was deleted).
EDIT: This video is about a Telugu program called Dharma Sandehalu which also talks about the same practice of wiping one's hands in front of Chandikeshvara. (Video courtesy- TheDestroyer)