Hide in Arthur's kingdom (4)
I believe the answer is:
skin
'hide' is the definition.
(a hide is an animal skin)
'in arthur's kingdom' is the wordplay.
'in' says the answer is hidden in the clue.
'SKIN' can be found hidden inside 'arthurs kingdom'.
(Other definitions for skin that I've seen before include "coat" , "Outer layer of tissue" , "Dermis" , "Surface layer" , "Remove the outer layer" .)