A king, held in reverence, is vigilant (5)
I believe the answer is:
awake
'vigilant' is the definition.
(wide awake)
'a king held in reverence' is the wordplay.
'king' becomes 'k' (abbreviation for king in chess).
'held in' means one lot of letters goes inside another.
'reverence' becomes 'awe' (synonyms).
'a'+'k'='ak'
'ak' put into 'awe' is 'AWAKE'.
'is' acts as a link.
(Other definitions for awake that I've seen before include "Patriotic song 'The West's .....'" , "Tossing and turning" , "Make active again" , "Completely conscious" , "No longer in the land of Nod" .)