The most common and most likely answer to this crossword clue is the 5 letter word ALERT
I've seen this crossword clue in The Guardian
A
L
E
R
T
More clues leading to the result ALERT
- Quick drink recruit drained
- Quick drink before road trip starts
- Awake, some lacklustre layabout turns over
- On the ball, shimmying later
- Advise caution right after beer
- Attentive dealer taking part
- Change, moving heart to foot – the way to stay in a 16 across, we’re told
- A warrant keeps Romeo on his toes
- What’s central to change is moving to end emergency?
- Arsenal gutted – Everton’s midfield is on the ball
Buddy explains!

The straight part
We know that "Attentive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case attentive = alert.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ALERT mean?
ALERT noun- An alarm.
- A notification of higher importance than an advisory.
- A state of readiness for potential combat.
- Attentive; awake; on guard.
- Brisk; nimble; moving with celerity.
- To give warning to.