Crossword Genius

Very slight force fluctuating in the raw outside (5-4)


I believe the answer is:

But I can't explain it at all!

(Other definitions for wafer-thin that I've seen before include "Having very little width or substance", "Very fine", "Very narrow", "Having little thickness", "Warn thief stealing slim biscuit - very slim".)

I've seen this clue in The Times.