Poor book promotion (3)
I believe the answer is:
bad
'poor' is the definition.
(I've seen this before)
'book promotion' is the wordplay.
'book' becomes 'b'.
'promotion' becomes 'ad' (ad is a kind of promotion).
'b'+'ad'='BAD'
(Other definitions for bad that I've seen before include "Ill" , "Of poor quality" , "Evil, below standard or off" , "Mouldy, off" , "Rotten, evil" .)