Run aground acting as sentry (2,5)
I believe the answer is:
on guard
'acting as sentry' is the definition.
I can't judge whether this definition defines the answer.
'run aground' is the wordplay.
'run' is an anagram indicator (I've seen this in other clues).
'aground' with letters rearranged gives 'ON GUARD'.
Can you help me to learn more?
(Other definitions for on guard that I've seen before include "Wary" , "Keeping watch" , "Alert like sentry" , "Watchful" , "Alert, vigilant" .)