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' (chess abbreviation).
'held in' is an insertion indicator.
'reverence' becomes 'awe' (synonyms).
'a'+'k'='ak'
'ak' put within 'awe' is 'AWAKE'.
'is' is the 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" .)