"Skulduggery" is an obscene book (5,4)
I believe the answer is:
dirty work
'skulduggery' is the definition.
(I've seen this before)
'an obscene book' is the wordplay.
'an obscene' becomes 'dirty' (I've seen this before).
'book' becomes 'work' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dirty'+'work'='DIRTY WORK'
'is' is the link.
Can you help me to learn more?
(Other definitions for dirty work that I've seen before include "Dishonest practice" , "Foul play" , "For example, what makes a clean sweep unlikely?" , "operation of sweep, say" , "Skulduggery" .)