Woman's rotten idea scaled down (4)
I believe the answer is:
enid
'woman's' is the definition.
'rotten idea scaled down' is the wordplay.
'scaled down' indicates the answer is hidden within the clue (the clue is cut down to just the answer).
'ENID' can be found hidden inside 'rotten idea'.
(Other definitions for enid that I've seen before include "Woman" , "- Blyton, twentith century writer" , "Geraint's wife's" , "Arthurian wife" , "Lady" .)