A thousand held in fear and wonder, vigilant (5)
I believe the answer is:
awake
'vigilant' is the definition.
(thesaurus)
'a thousand held in fear and wonder' is the wordplay.
'thousand' becomes 'k' ('K' can be a synonym of 'thousand').
'held in' means one lot of letters goes inside another.
'fear and wonder' becomes 'awe' (I've seen this before).
'a'+'k'='ak'
'ak' put into 'awe' is 'AWAKE'.
(Other definitions for awake that I've seen before include "Conscious, aware" , "Become active again" , "Tossing and turning" , "Conscious and (usually) open-eyed" , "Cease sleeping" .)