It's no good using second-class publicity (3)
I believe the answer is:
bad
'it's no good' is the definition.
(similar in meaning)
'second-class publicity' is the wordplay.
'second-class' becomes 'b' ().
'publicity' becomes 'ad' (short for advert).
'b'+'ad'='BAD'
'using' acts as a link.
(Other definitions for bad that I've seen before include "Ill" , "Rotten, evil" , "Mouldy, off" , "Substandard" , "Spoiled or evil" .)