Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† mĕsŏmăcros, i, m., = μεσόμακρος (sc. pes), a poetical foot of five syllables, of which only the middle one is long (e. g. avidissimus; opp. mesobrachys), Diom. p. 478 P.