Terrible British commercial (3)
I believe the answer is:
'terrible' is the definition.
(similar in meaning)
'british commercial' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'commercial' becomes 'ad' (abbreviation for advertisement).
(Other definitions for bad that I've seen before include "Stinking" , "SECOND HALF OF 16" , "not healthy" , "Rotten, evil" , "Mouldy, off" .)