Be on fire (4)
I believe the answer is:
burn
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">be on fire</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for burn that I've seen before include "Use as fuel" , "be consumed" , "Go up in smoke" , "Small stream in Scotland" , "Smart" .)