Reckless royal officer briefly disrupting plan (6)
I believe the answer is:
madcap
'reckless' is the definition.
(thesaurus)
'royal officer briefly disrupting plan' is the wordplay.
'royal officer briefly' becomes 'adc' (abbreviation for aide-de-camp).
'disrupting' means one lot of letters goes inside another (inside letters 'disrupt' the flow of outside one).
'plan' becomes 'map' (I've seen this before).
'adc' going into 'map' is 'MADCAP'.
(Other definitions for madcap that I've seen before include "Oddball" , "Crazy fellow" , "Amusingly eccentric" , "Hothead" , "Reckless, hot-headed" .)