Pornographic material is mounted at prime locations (4)
I believe the answer is:
smut
'pornographic material' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for smut that I've seen before include "black spot" , "blue material" , "Dirty stuff" , "Disease of cereal grass" , "Cereal disease; filth" .)