Homophone clue
A homophone clue is a cryptic crossword clue in which the answer sounds like a different word, and an indicator tells the solver to listen to the clue rather than read it.
The indicator is always some reference to speech or hearing: we hear, reportedly, said, aloud, on the radio, announced, so to speak, by the sound of it. Without one the clue is unfair, because nothing else in a written grid would tell you to try pronouncing a word.
The clue "Two, by the sound of it, also (3)" gives TOO. The wordplay is TWO said aloud, the indicator is by the sound of it, and also is the definition. Both halves land on the same three letters.
Homophones are the most argued-over device in cryptic crosswords, because whether two words sound alike depends on the accent doing the sounding. Setters in Britain will happily clue a pairing that an American solver hears as two different words, and the reverse happens just as often. Editors usually restrict homophones to pairings that hold across most accents, and solvers still complain.