What is this Pasuk teaching us?
Rashi (citing the Sifra): It teaches us that a Kohen may not render himself Tamei for a wife who defiled him from his Kehunah whilst she was alive, 1 provided there is someone to bury her - she is not a Meis Mitzvah.
Ramban, Seforno and Targum Onkelos: It means that a master among his people 2 (a Kohen) should not desecrate his Esteemed position through Tum'as Meis. 3
Rashbam: It means that a Kohen is not permitted to render himself Tamei for his deceased wife, because it desecrates his Kehunah. 4
Targum Yonasan #2: It implies that a Kohen may render himself Tamei for relatives only if they have not left the fold. 5
Hadar Zekenim: Even a master in his nation - the Kohen Gadol may not render himself Tamei for him; Da'as Zekenim - it is permitted only if he is his father, brother or son.
Moshav Zekenim (citing Sifra 6 ): When the Meis is among the nation (not a Meis Mitzvah), he may not render himself Metamei for it.
Nazir, 43a: #1: "Leheichyalo" implies that the prohibition against Tum'as Meis only applies after the person actually dies - but not to a Goseis. 7
Nazir, 43a #2: "Leheichalo" confines the Chiyuv Malkos to a Tahor Kohen who renders himself Tamei - to preclude a Kohen who is Tamei already for a relative. 8
As we will learn in Pasuk 7. The Gemara in Yevamos, 22b, learns this from the apparent contradiction between Pasuk 1 - which permits a husband to render himself for Tamei for his deceased wife - and this Pasuk, which prohibits it.
Seforno: It is therefore befitting that he should behave in an aristocratic fashion so that he commands the respect of the people, confining his dealings with Meisim to members of his family, since their Kavod is his Kavod too (See Seforno). Ramban: It denotes importance, as in Shmuel 2, 6:2, Yeshayah, 16:8 and Sh'mos, 22:13. See also Oznayim la'Torah DH 'Lo Yitama ... ' as to why the Ramban delines to learn like Rashi (the Sifra).
Even when he is not serving in the Beis-ha'Mikdash.
I.e. the simple meaning of the Pasuk applies to any wife. Rashbam points out that Chachamim expounded that the Isur is only for a Pasul wife. (PF)
See Torah Temimah, notye 28.
Our text of Sifra learns this from Pasuk 1! It seems that Moshav Zekenim's text learns from this Pasuk.
Even though the maority of Gos'sim die (Torah Temimah).
See Torah Temimah, note 30. The Gemara there, 42b makes a similar D'rashah with regard to a Kohen Gadol based on the words "ve'Lo Yechalel" in Pasuk 12.