Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
circum-sĕcus, adv., round, round about, in the region around (only in App.): astantes, App. M. 2, p. 121, 11; 5, p. 166, 23; 11, p. 264, 33; cf. Hand, Turs. II. p. 74.