Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
obreptīvus, a, um, adj. [obrepo], secret, clandestine (post-class.): supplicatio, Symm. Ep. 5, 64; Cod. Th. 16, 10, 8.
Hence, adv.: obreptīvē, surreptitiously, clandestinely (post-class.): aut clanculo supplicare, Cod. Th. 16, 1, 4 fin.