Down or dirty (4)
I believe the answer is:
blue
I believe this clue is a double definition.
'down' is the first definition.
(both can mean sad)
'dirty' is the second definition.
(blue can mean pornographic)
(Other definitions for blue that I've seen before include "Dejected" , "Tory?" , "Azure" , "entertaining rabbi" , "Sad; university sportsman" .)