Harry's son that is unobserved being mischievous (4)
I believe the answer is:
arch
'mischievous' is the definition.
(arch can mean mischievous or playful)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for arch that I've seen before include "Affectedly playful in curved manner" , "Inner part of foot" , "Prefix meaning chief - maybe an angel" , "Roguish - chief - supporter" , "Curved span" .)