The Green Bully Pulpit

Plausible(adj.)
Synonym: credible
Antonym: fabrication
Sentence: "A plausible explanation is that McCain sought to avoid taking a position that would offend either conservative primary voters or the moderate ones he will need in November."
Definition: Having an appearance of truth or reason; seemingly worthy of approval or acceptance; credible; believable (Dictionary.com)
Context Clues: By reading the paragraph i could tell what the word meant. Explanation is what truly gives it away.

Extemporaneously(adj.)
Synonym: improvised
Antonym: prepared
Sentence: His speech was brilliant -especially considering that it was so hastily prepared that the last seven minutes were delivered extemporaneously.
Definition: Spoken, performed, or composed with little or no preparation or forethought. (Dictionary.com)
Context Clues: I initially thought that extemporaneously meant brilliant or fantastic, but i re-read the sentence again and came up with the actual meaning.

Tenacity(adj.)
Synonym: hardheaded
Antonym: yielding
Sentence: Hillary Clinton has gritty tenacity and long experience dealing with balky politicians.
Definition: holding fast; characterized by keeping a firm hold;holding or tending to hold persistently to something, such as a point of view(Dictionary.com)
Context Clues: When the sentence said balky politicians i knew that you hand to stand firm on your ground somehow so i got the definition with no problem.