Q. How do I cite something my source has cited in MLA? It's a source within a source.

MLA (8th edition) calls this "citing an indirect source." The source that you used would be the entry for the bibliography. In the in-text or parenthetical reference put "quoted in" before the indirect source you cite (if what you cite is a direct quote). See the MLA Handbook, section 3.4, page 124 for more information.

In-Text example:
Mary Sutter Thomas revealed in her diary that she had "concocted the adventure" (qtd. in Peterson 79).

Note:  qtd. in = quoted in