Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. immissus (inm-), a, um, Part., from immitto.
* 2. immissus (inm-), ūs, m. [immitto], a letting in, admission: sol cuncta lustrat lucis immissu, Macr. S. 1, 18, § 11.