Royal aide in palace ultimately doubt, king admitted (7)
I believe the answer is:
equerry
'royal aide' is the definition.
(I've seen this before)
'palace ultimately doubt king admitted' is the wordplay.
'ultimately' says to take the final letters.
'doubt' becomes 'query' (I've seen this before).
'king' becomes 'r' (r is abbreviation for rex).
'admitted' is an insertion indicator.
The last letter of 'palace' is 'e'.
'query' enclosing 'r' is 'querry'.
'e'+'querry'='EQUERRY'
'in' acts as a link.
(Other definitions for equerry that I've seen before include "Attendant at palace" , "Royal aide" , "Royal family official" , "Officer attending a royal family member" , "Officer of the British royal household" .)