one tanka; one poem
Counter for tanka (31-syllable Japanese poems) and occasionally other short poems. Often used in contexts like poetry contests or collections.
He composes one tanka every day.
I composed one poem while traveling.
She composed a tanka to celebrate the wedding.
The judges chose one tanka from among them.
一首 is the counter used for one tanka (a 31-syllable poem), while 一句 counts one haiku or similarly short verse.
Hyakunin Isshu; classical anthology of 100 waka by 100 poets · hyakunin isshu karuta; poetry card game
Compound of 一 (one) and 首 (counter for poems). The use of 首 as a counter for poems is ancient, originally referring to the 'head' or beginning of a poem.