### Clue: Unruly youngster starts on telescope? (10)
### Answer: REFRACTORY 
 ### Explanation: Definition: Unruly
 **The 1st letter of**(… starts) "youngster " **placed after**(on, in an across clue) REFRACTOR(a telescope that uses refracting lenses).
The clue could have been better with "… youngsters start …" to make the wordplay unambiguous.