Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
subsessa, ae, f. [subsido], an ambush, ambuscade (post-class.): subsessas occultius collocare, Veg. Mil. 3, 6 med.: in subsessā esse, id. ib. 3, 22; cf.: hostium dolos subsessas vocamus, Serv. Verg. A. 11, 268.