Vigilance later affected head (9)
I believe the answer is:
alertness
'vigilance' is the definition.
(synonyms)
'later affected head' is the wordplay.
'affected' indicates anagramming the letters.
'head' becomes 'ness' (ness is a headland).
'later' is an anagram of 'alert'.
'alert'+'ness'='ALERTNESS'
(Other definitions for alertness that I've seen before include "Watchful state" , "It's needed to respond quickly to danger" , "Watchful attitude" , "State of vigilance and watchfulness" , "Showing attentive nature" .)