Ad (5)
(Other definitions for hoc that I've seen before include "Ad ..., for one particular purpose only" , "For Romans, this" , "Ad - means for a single purpose only" , "Ad ..., for one particular situation only" .)
'ad' is the definition.
(I know that promo is a type of ad)
This is the entire clue.
(Other definitions for promo that I've seen before include "publicity event" , "Advertising film or video" , "21 [TRAILER]" , "Publicity for project or product (abbr)" , "Ad" .)