The headword is aorist active imperative, second person plural, of the verb klh/|zw (cf. kappa 1773 and LSJ s.v., I). Evidently quoted from somewhere -- but not Euripides, Alcestis 548 (web address 1), where the verb in question is klei/w (cf. kappa 1775); rather, probably, Aristophanes, Birds 1745 (web address 2).
cf. also ps.-Zonaras s.v. klhi+/zw, Etymologicum Magnum s.v. klh/zw and Hesychius s.v. klhizo/menos.
[1] Same form of the verb u(mne/w; cf. upsilon 112 and upsilon 114.