or, if you know your filenames are all going to have that structure with no digits apart from the 8-digit ymd date:
test_out <- list.files(here::here("./data/clean"), pattern = "RDS$") %>%
dplyr::as_tibble() %>%
dplyr::mutate(mdate = stringr::str_extract(value, "[:digit:]+"))
dplyr::slice_max(order_by = mdate) %>%
dplyr::pull(value) %>%
here::here("./data/clean", .) %>%
readRDS(.)
(amend regex as necessary depending on filename complexities)