The headword is aorist indicative active, third person singular, of sfa/llw, a verb which can have both intransitive ('fall') and transitive ('cause to fall') meanings; see LSJ s.v.; the latter, underlined by the supplementary gloss, is what is illustrated here; cf. sigma 1716. The headword might be extracted here from the quotation appended, but there are notable alternatives (e.g. Thucydides 2.87.3).
[1] Likewise in Hesychius (who also gives the 'made to topple' gloss) and other lexica.
[2] Arrian, Parthica fr. 22 (preserved only here). For Crassus cf. alpha 2721, pi 30.