Vigilant, but breaking down later (5)
I believe the answer is:
alert
'vigilant' is the definition.
(similar in meaning)
'breaking down later' is the wordplay.
'breaking down' indicates anagramming the letters.
'later' is an anagram of 'ALERT'.
'but' acts as a link.
(Other definitions for alert that I've seen before include "'Attentive, vigilant (5)'" , "Watchful; warning" , "Signal danger" , "Watchful, quick-thinking" , "Watchfully awake" .)