The clue means the reverse. Easy to solve, easy to solve backwards.
An opposites clue points at a word by naming its opposite. A tile reading "out" is hiding IN. A tile reading "small" is hiding BIG. It is the simplest transformation in the game and it produces more self-inflicted wrong guesses than any other.
The reason it exists is the same reason direct clues exist: popular music is built out of very small words, and small words are hard to clue interestingly. But where a direct clue surrenders and hands the word over, an opposites clue finds the one genuinely fair thing you can say about a word like "on" without saying it. Every small word has an opposite, and the opposite is always exactly as short and exactly as ordinary.
There is something appealing about how much these clues depend on the listener rather than the writer. The clue "you" points at ME, but only because you understand that you and me are a pair. Nothing in the letters suggests it. The relationship is entirely conceptual and completely obvious, which is a rare combination.
They are especially common in titles built on contrast and direction, which is why they concentrate in particular decades. When titles are short and turn on a preposition, the opposites clue is often the only honest tool available.
Cryptune contains 155 opposites clues, which is 9% of every clue in the game, spread across 130 puzzles. They break down as 99 on song puzzles, 43 on album puzzles, 13 on artist puzzles.
out→IN
From In Rainbows. The purest example in the game. One word, one reversal, and precisely the trap described above.
you→ME
From Call Me Maybe. Pronouns are opposites only by convention, but the convention is so strong that the clue reads as obvious. Compare it with the reverse case below.
me→YOU
From Wish You Were Here. The same pair running the other way. When you meet one of these, the immediate question is which direction the title needs, and the answer is rarely deducible from the clue alone.
do→DONT
From Dont Stop Believin. A negation rather than a true opposite, and a good demonstration that the category is broader than antonyms. Anything that flips the sense qualifies.
A very short, very ordinary clue that is not a plausible synonym for anything is usually an opposite. If a tile just says "off" and no synonym presents itself, try reversing it before anything else.
The pronoun pairs are worth memorising because they recur constantly. You and me. My and your. She and he. In a game full of first person and second person titles, these carry an enormous amount of the small vocabulary.
Then check your answer twice. This is the one clue type where solving correctly and writing the wrong word are the same motion. You decode "out", you know the answer is IN, and your fingers type "out" because that is the word you have been looking at. It happens to everyone.
A wider sample, clue on the left and the word it hides on the right.
Back to the clue school, or see how clue types vary across the eras on the decade pages.