The headword kludw/nion is a grammatical diminutive of klu/dwn, "billow/wave", attested in Attic tragedy and elsewhere; see generally LSJ s.v. (web address 1 and web address 2), and nn.2-3 below.
[1] Also a gloss at kappa 2635 -- but here, as has been realised, it cannot be right; it is a mistake for the noun ku=ma (Photius, Lexicon kappa798 Theodoridis, etc.).
[2] (The implicit le/gousi is explicit in Photius kappa799, on which see further, next note.) This second noun is here in the accusative case klu/dwna, so evidently the headword is too.
[3] The codices of Thucydides 2.84.3 have kludwni/w|, i.e. the dative case of the present headword (web address 3). Nevertheless, editors of Thucydides have preferred to follow the reading of the present entry, also in Photius: klu/dwni. (According to the TLG, this is the only use of the word klu/dwn in Thucydides.)
[4] Only in lexica and grammars; LSJ entry at web address 4.