Royal Frederick is only partially an English king (6)
I believe the answer is:
alfred
'an english king' is the definition.
'Alfred' can be an answer for 'king' (I've seen this before). I'm unsure of the 'an english' bit.
'royal frederick is only partially' is the wordplay.
'is only partially' indicates the answer is hidden within the clue.
'ALFRED' is hidden in the letters of 'royal frederick'.
Can you help me to learn more?
(Other definitions for alfred that I've seen before include "Cake-burning king" , "A great king of Wessex, d. 899" , "Royal one across burner" , "Ninth century king of Wessex" , "English-born film director" .)