Egghead with question about king's attendant (7)
I believe the answer is:
equerry
'attendant' is the definition.
(equerry is a kind of attendant)
'egghead with question about king's' is the wordplay.
'egghead' becomes 'eery' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' is an insertion indicator.
'question' becomes 'qu'.
'about' is an insertion indicator.
'king' becomes 'r' (r is abbreviation for rex).
'eery' going around 'qu' is 'equery'.
'equery' placed around 'r' is 'EQUERRY'.
Can you help me to learn more?
(Other definitions for equerry that I've seen before include "Attendant at court" , "Officer of the British royal household" , "Attendant in royal household" , "Royal aide" , "Personal attendant of a member of the royal family" .)