1. A group hired to applaud at a performance.
2. A group of fawning admirers or sycophants.
Etymology: mid-1800s, French claquer, to clap.
Wikipedia has an article on the original claquers (groups of professional applauders).
This is one of those words that, the longer I stare at it, the more mis-spelled it looks.
It's unrelated to 'clique' (a small exclusive group), although both originated from French verbs (cliquer, to clink or make noise).