ato usu wa karasu no mochi ka saiho^ji
is the next batch of rice cakes
for the crow?
Shinji Ogawa notes that ato usu means "the next batch of rice cakes" (not, as I originally thought, the "tub in back"). Issa makes a statement instead of a question: "The next batch of rice cakes is for the crow" (ato usu wa karasu no mochi ya). There are two types of usu or mill: (1) shiki usu (grinding hand-mill) and (2) a large wooden tub used for rice or herb cake making. The cake maker pounds the ingredients with a wooden mallet. The second definition fits here.
Not: Saiho Temple by Gavi