Insofar as the question is asking about scenarios where the word is, say, "pizza" and you give a "P" to indicate that the word starts with P, I think it would be allowed by the
rules. However, I'd argue that it'd be in bad form, even though I think it'd technically be allowed. Aghagh seems to think it wouldn't be allowed in that instance. But I am not sure what the argument is for that. The official rules excplitly say that: "A proper noun (Sherlock, Lego), compound word (merry-go-round), number (007), onomatopoeia (Riiing), acronym (FBI), or special character ($) are all considered to be valid clues." If a special character or a number can be valid, then I'd assume a letter would be as well, barring the instance aghagh mentions where the letter is a homophone (e.g., B for Bee). The rules also provide what seems to be an exhaustive list of invalid clues, and the starting letter is not listed. According to the rules, invalid clues are as follows:
• The Mystery Word written in a different language (Vert to guess Green)
• A word in the same family as the Mystery Word (Princess to guess Prince)
• A made-up word (Cuppajo to guess Coffee)
• A homophone of the Mystery Word (Flour to guess Flower)
Beyond homophones and starting letters, I agree with aghagh that there are scenarios where a single letter is not only clearly allowed but also very appropriate and in good form. For instance, when the mystery word was "Metro", I gave the clue "L" because the L is a common name for Chicago's metro.