Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
summissĭo (subm-), ōnis, f. [summitto], a letting down, lowering, dropping, sinking (Ciceron.): ex contentione vocis et summissione, Cic. Off. 1, 41, 146: (iterationes) erunt ab hac summissione orationis alienae, id. Or. 25, 85: nec elatio nec summissio, i. e. depression, id. Top. 18, 71.