The term 'O in a phonetic alphabet' is referring to the letter 'O' in the International Phonetic Alphabet (IPA). The IPA is an alphabetic system of phonetic notation based primarily on the Latin alphabet. It is used by linguists to accurately and uniquely represent each of the wide variety of sounds used in spoken language. The solution to this crossword is 'OBOE', which is a woodwind instrument that produces a mellow, reedy sound.