Clue: Queen of mystery: queen wearing boot but no cap (6)
Answer: ELLERY
Explanation: Definition: Queen of mystery
An envelope ('wearing') of ER ('queen', the second one in the clue) in [w]ELLY (Wellington 'boot') minus the first letter ('but no cap'). The first 'queen', in the definition, is not female royalty, but Ellery Queen, the fictional character in mystery stories by Frederic Dannay and Manfred Bennington Lee.