What you describe could be (tentatively) called a "semantic singularization of plural words", i.e. the fact that plural words may, by assimilation drift to a singular meaning. Since the "-a" form in plural is not common in English, and rather sounds like a singular, it is all the more likely to occur.
Hence indicia, plural form ultimately from Latin index. The word also exists in English, and does its job as a singular of indicia:
Something that reveals or indicates; a sign: "Her face ... was a fair
index to her disposition" (Samuel Butler)..
According to American Heritage it is "Markings on bulk mailings used as a substitute for stamps or cancellations." If your definition uses a singular it is either some kind of simplication by the person who wrote the definition, or a bastardization (which could be considered the same thing).
Note that you have a similar issue with criterion/criteria where the form criteria has been used as a singular for some time.