Sudden clever idea (5,4)
(Other definitions for brainwave that I've seen before include "Good idea" , "Great idea" , "Sudden bright idea" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sudden clever idea</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for brain wave that I've seen before include "Brilliant notion" , "Sudden good idea" , "A sudden inspiration or bright idea" , "Sudden clever idea" , "Brilliant idea" .)