### Clue: Radical junior in charge too high to catch? (10)
### Answer: ULTRASONIC 
 ### Explanation: Definition: too high to catch
 ULTRA(a radical/an extremist) + SON(general term for a young person/a junior) + IC(abbrev. for "in charge").
Defn: …/describing a sound wave whose frequency is above/too high for the human ear to perceive/catch.