Sudden clever idea (9)
(Other definitions for brainwave that I've seen before include "Good idea" , "Great idea" , "Sudden bright idea" .)
'sudden clever idea' is the definition.
(I've seen this before)
This is all the clue.
(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" .)