אמר רבא פשיטא לי נר ביתו ונר חנוכה נר ביתו עדיף משום שלום ביתו
Says Rava: It is obvious to me, that if its a choice between light for his house (on Shabbos - see Rashi) or Channukah light, then the light for his house is more important because of peace in the home.
This is also brought as the accepted Halachah in Shulchan Aruch Orach Chaim 678.
It should be noted that it means minimum. So, if someone had 2 candles, they would split 1 for Shabbos and 1 for Channukah. (not two for shabbos as is customary-see Kaf HaChaim)
However, the Kaf HaChaim says that if a poor man has only 1 candle, it may double as both the Shabbos candle and the Channukah candle at the same time. He claims that in such a difficult situation of poverty, the usual prohibition of not being allowed to benefit from the light is lifted for the sake of the poor man. So he can eat the meal by the light of the Channukah candle in this case. (Kaf HaChaim quoting the Magen Avraham)
So in that case, the obligation to sell the clothing would apply so he could buy 1 Channukah candle, since it would do double service as his Shabbos light.
(Perhaps if he had money for just one candle, he would be spared selling his clothing because he could buy one candle and use it for both Shabbos and Channukah as well.)
He bases this on the fact that in the times of the Gemara, the mitzvah was to light near the street. Therefore, it would never dawn on Rava to say the Menorah could accomplish double service for a poor man on Shabbos (since the Channukah light needed to be in the street, while the Shabbos light had to be in the home). However, later authorities ruled that the obligation to light Menorah is fulfilled in one's home, not facing the street (See S.A. 671:7 based on the opinion of the Rema there).
Therefore, there is room to say the prohibition to use the light would be lifted just as the Gemara allows one to use the light at a time of danger.
The Menorah may be put on one's own table (in the home, and not the street, during times of religious persecution,) and its enough.
The Ran asks how you can put it on your table when you will certainly come to use it. The Ran answers that in such a difficult case, the prohibition to make use of the Channukah lights is lifted.
Further support for this is taken from Shulchan Aruch 673 which brings an opinion that holds one may make use of the Channukah lights for the sake of a mitzvah. (Shabbos light and shalom bayis for the sake of celebrating Shabbos, qualify as a mitzvah.)
However, although some agree with the Kaf HaChaim who says this understanding is possible, not all agree.
In any case, it would seem that a poor man relying on the Magen Avraham would place this one candle in a new location on his table to remind everyone that it is a Channukah light and not just a Shabbos light.
(By the way: Nowadays, if the person had an electric light source, it would work as a Shabbos candle but not a Channukah candle. So money would be spent on a Channukah candle while flipping on the lightbulb before Shabbos, for Shabbos light.)