Alert - run aground (2,5)
I believe the answer is:
on guard
'alert' is the definition.
(thesaurus)
'run aground' is the wordplay.
'run' indicates anagramming the letters (I've seen this in other clues).
'aground' is an anagram of 'ON GUARD'.
(Other definitions for on guard that I've seen before include "Wary" , "Alert like sentry" , "Alert, vigilant" , "Keeping watch" , "Watchful" .)