Clue: Unusual sonata, nothing to be missed, this person's penned? (7)
Answer: SMETANA
Explanation: Unusual sonata, nothing to be missed, this person's penned? (7)
An envelope ('penned') of ME ('this person') in STANA, an anagram ('unusual') of 's[o]nata' minus the O ('nothing to be missed'), with an extended definition.