。
After climbing Mount Fuji, a poem came to me.
Literal
Fuji-mountain [to-に] climbed after [with-で] one-poem floated.
一首 is the counter for waka/tanka poems (31-syllable form). 浮かんだ ('floated up') is the standard verb for ideas or poems coming to mind spontaneously — the image is of something rising to the surface. 後で ('after') marks the temporal sequence. The sentence evokes the classical tradition of composing poetry inspired by nature and travel.