Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ef-fervo, ĕre (praes. effervent, Vitr. 2, 6, 5), v. n., to boil up or over (poet. and in post-Aug. prose): in agros Aetna, Verg. G. 1, 471: quatenus in pullos animales vortier ova Cernimus alituum vermesque effervere, qs. to come boiling forth, i. e. to swarm forth, Lucr. 2, 928; cf. Verg. G. 4, 556; id. Dir. 15; Stat. Th. 4, 664.
Hence, effer-vens, entis, P. a., boiling with passion, i. e. fervent, ardent: siquidem laetitia dicitur exsultatio quaedam animi gaudio efferventior eventu rerum expetitarum, Gell. 2, 27, 3.