Ogre round lake after six is watchful (8)
I believe the answer is:
vigilant
'watchful' is the definition.
(synonyms)
'ogre round lake after six' is the wordplay.
'ogre' becomes 'giant' (I've seen this before).
'round' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'after' is a charade indicator (letters next to each other).
'six' becomes 'VI' (Roman numerals).
'giant' placed around 'l' is 'gilant'.
'gilant' after 'vi' is 'VIGILANT'.
'is' acts as a link.
(Other definitions for vigilant that I've seen before include "On the lookout" , "Ever watchful" , "Watchful, on guard" , "Keeping careful watch" , "Be watchful while I gilt van" .)