Alert, but careless around about the third of August (2,5)
I believe the answer is:
on guard
'alert' is the definition.
(I've seen this in another clue)
'careless around about the third of august' is the wordplay.
'careless' is an anagram indicator.
'about' is an insertion indicator.
'the third of august' becomes 'g' (3rd letter of 'august').
'around' is an anagram of 'onuard'.
'onuard' placed around 'g' is 'ON GUARD'.
'but' is the link.
(Other definitions for on guard that I've seen before include "Alert like sentry" , "Wary" , "Alert, vigilant" , "Keeping watch" , "Watchful" .)