Price to enter publicity campaign? (9)
I believe the answer is:
admission
'price to enter' is the definition.
(cost of admission)
'publicity campaign?' is the wordplay.
'publicity' becomes 'ad' (short for advert).
'campaign?' becomes 'mission' (campaign can mean a task or mission).
'ad'+'mission'='ADMISSION'
(Other definitions for admission that I've seen before include "Disclosure" , "entry charge" , "guilty revelation" , "Ticket holders expect this" , "Entrance fee" .)