Midrasz do Kapłańska 22:7

(Gemara) Let us see: when do the priests enter to eat the Terumah? Is it not when the stars appear? Let then the Mishnah say: "From the time the stars appear!" In using this expression, he lets us hear something by the way; namely, that with the appearing of the stars, the priests are allowed to eat their Terumah, because the forgiveness-offering [which will be brought on the morrow] is not a hindrance, as we have been taught; "And when the sun hath set, he shall be clean. (Lev. 22, 7.) i. e., the waiting for the setting of the sun prevents him from eating the Terumah, but not his forgiveness-offering." (Ib. b) R. Jose said: "Twilight lasts as long as a twinkling; this one comes and that one goes, and it is impossible to determine its exact time."

(Gemara) Let us see: when do the priests enter to eat the Terumah? Is it not when the stars appear? Let then the Mishnah say: "From the time the stars appear!" In using this expression, he lets us hear something by the way; namely, that with the appearing of the stars, the priests are allowed to eat their Terumah, because the forgiveness-offering [which will be brought on the morrow] is not a hindrance, as we have been taught: "And when the sun hath set, he shall be clean" (Lev. 22:7.) i.e., the waiting for the setting of the sun prevents him from eating the Terumah, but not his forgiveness-offering.

2) "until he is clean": I might think, until he immerses; it is, therefore, written (Vayikra 22:7) "And when the sun sets, he shall be clean." Just as his being clean below is with the setting of the sun, so, his being clean here.