Screen dog ain't mad (7)
I believe the answer is:
curtain
'screen' is the definition.
(I know that curtain is a type of screen)
'dog ain't mad' is the wordplay.
'dog' becomes 'cur' (both can mean a despicable person).
'mad' indicates anagramming the letters.
'aint' with letters rearranged gives 'tain'.
'cur'+'tain'='CURTAIN'
(Other definitions for curtain that I've seen before include "Hanging drapery for window" , "Hanging, window cover" , "Final instruction of play" , "Hanging material screen" , "It can be drawn" .)