poppycock (11)
(Other definitions for codswallop that I've seen before include "Hogwash" , "waffle?" , "Baloney" , "[BUNKUM]" , "Nonsense (colloq.)" .)
'poppycock' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rot that I've seen before include "bunk" , "crumble" , "Deterioration" , "Decay - nonsense" , "Cobblers" .)
(Other definitions for my eye that I've seen before include "Bull" , "bunk?" , "What rubbish!" , "expression of disagreement" , "rhyme" .)
(Another definition for foolishness that I've seen is " Folly".)