Happening, brewing (5)
I believe the answer is:
afoot
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">happening</span>' is the first definition.<br/ ><span class="explanation-format__explanation">('something's afoot' means 'something's happening')</span><br/ ><br/ >'<span class="explanation-format__original">brewing</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for afoot that I've seen before include "in the air" , "Astir" , "Under way (like hunted game?)" , "In the pipeline" , "Progressing, in operation - like Sherlock Holmes's game" .)