Watchfulness (9)
(Other definitions for vigilance that I've seen before include "Keeping careful watch on" , "Alertness, watchfulness" .)
'watchfulness' is the definition.
(synonyms)
This is all the clue.
(Other definitions for alertness that I've seen before include "Wakeful characteristic" , "Showing attentive nature" , "It's needed to respond quickly to danger" , "Watchful state" , "State of vigilance and watchfulness" .)