Vigilant, a king in dread (5)
I believe the answer is:
awake
'vigilant' is the definition.
(thesaurus)
'a king in dread' is the wordplay.
'king' becomes 'k' (**).
'in' means one lot of letters goes inside another.
'dread' becomes 'awe' (I've seen this before).
'a'+'k'='ak'
'ak' put inside 'awe' is 'AWAKE'.
(Other definitions for awake that I've seen before include "Completely conscious" , "Unable to sleep" , "Stop sleeping" , "Conscious and (usually) open-eyed" , "Make active again" .)