Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sŭb-ămārus, a, um, adj., somewhat bitter, bitterish (Ciceronian), Cic. Fat. 4, 8.
Adverb.: subamarum arridens, somewhat bitterly, Amm. 27, 11, 5.