Harry's son that is unobserved being mischievous (4)

I believe the answer is:

arch

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'mischievous' is the definition.
(arch can mean mischievous or playful)

I don't understand the rest of the clue.

(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" .)

I've seen this clue in The Telegraph.

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