Billow (5)
I believe the answer is:
surge
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">billow</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for surge that I've seen before include "significant increase" , "Upward rush" , "Sudden onward motion" , "Increase rapidly in intensity" , "Rolling swell of the sea" .)