Six huge guards left keeping watch (8)
I believe the answer is:
vigilant
'keeping watch' is the definition.
'six huge guards left' is the wordplay.
'six' becomes 'vi' ('vi' can be a synonym of 'six').
'huge' becomes 'giant' (I've seen this before).
'guards' is an insertion indicator.
'left' becomes 'l' (common abbreviation).
'giant' enclosing 'l' is 'gilant'.
'vi'+'gilant'='VIGILANT'
(Other definitions for vigilant that I've seen before include "Keeping careful watch" , "Ever watchful" , "Watchful, alert to possible danger" , "On the lookout" , "Carefully observant or attentive" .)