Clever like King, briefly Egypt's leader (6)
I believe the answer is:
astute
'clever' is the definition.
(I've seen this before)
'like king briefly egypt's leader' is the wordplay.
'like' becomes 'as'.
'king briefly' becomes 'tut' (King Tut, short for Tutankhamun; 'briefly' suggests a short form).
'leader' indicates taking the first letters.
The first letter of 'egypts' is 'e'.
'as'+'tut'+'e'='ASTUTE'
(Other definitions for astute that I've seen before include "Shrewd and sagacious" , "Clever and perceptive" , "smart (with some self-interest?)" , "Discerning" , "Peerceptive" .)