Influx (6)
I believe the answer is:
inrush
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">influx</span>' is the definition.<br/ ><span class="explanation-format__explanation">(inrush is a kind of influx)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for inrush that I've seen before include "Sudden inward flow" , "Inflow" , "An inward dash" , "Crowd invasion" , "Sudden arrival (of people or water?)" .)