According to Harpokration s.v., the Suda's source, this verb was used in Demosthenes 21, Against Meidias. It does not appear in any of the manuscripts of that speech, but is a conventional emendation of the second of a pair of such verbs - "hiss and ..." - which feature in ch. 226 there.
In Harpokration s.v. (and hence in Demosthenes 21.226: web address 1) it is imperfect active, second person plural. In the Suda this has become imperfect middle, third person singular - an accidental change ignored in translation here.
What kind of sound klw/zein was is not clear either from this Demosthenic passage or from Alciphron 3.71 (where again it is paired with "hiss", suri/ttein). Pollux 5.89 includes it in a list of vocabulary for bird calls, as being that of a jackdaw, but MacDowell (below) 422-3 calls this "not much help. We can only say that it was a sound conventionally used to show disapproval, equivalent in intent to a modern hoot or boo but not necessarily the same phonetically."