On the ball; warning (5)
I believe the answer is:
alert
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">on the ball</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(on the ball can mean alert or paying attention)</span><br/ ><br/ >'<span class="explanation-format__original">warning</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that alert is a type of warning)</span><br/ ><br/ ></span>
(Other definitions for alert that I've seen before include "Watchful and bright" , "Warning signal" , "'Attentive, vigilant (5)'" , "Put on guard, warn" , "On one's toes" .)