Skulduggery in the obscene book? (5,4)
I believe the answer is:
dirty work
'skulduggery' is the definition.
(I've seen this before)
'obscene book?' is the wordplay.
'obscene' becomes 'dirty' (similar in meaning).
'book?' becomes 'work' (book is a kind of work).
'dirty'+'work'='DIRTY WORK'
'in the' acts as a link.
(Other definitions for dirty work that I've seen before include "Dishonourable dealings - need bath afterwards" , "operation of sweep, say" , "Foul play" , "Skulduggery" , "For example, what makes a clean sweep unlikely?" .)