Watchful as at six, huge lake breached in (8)
I believe the answer is:
vigilant
'watchful' is the definition.
(synonyms)
'at six huge lake breached in' is the wordplay.
I cannot really understand how this works, but
'six' could be 'vi' (roman numeral) and 'vi' is found within the answer.
'lake' could be 'l' and 'l' is found in the answer.
'huge' could be 'giant' (I've seen this before) and 'giant' is found within the leftover letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).
'as' acts as a link.
Can you help me to learn more?
(Other definitions for vigilant that I've seen before include "Be watchful while I gilt van" , "Keeping careful watch" , "On the lookout" , "Carefully observant or attentive" , "Watchful, alert to possible danger" .)