Down and dirty (4)
I believe the answer is:
blue
This clue is a double definition.
'down' is the first definition.
(both can mean sad)
'dirty' is the second definition.
(blue can mean pornographic)
'and' is the link.
(Other definitions for blue that I've seen before include "in Oxford, it's dark" , "Unsuitable for young" , "Sad; butterfly" , "adult" , "very cold" .)