Terrible British commercial (3)
I believe the answer is:
bad
'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).
'b'+'ad'='BAD'
(Other definitions for bad that I've seen before include "Stinking" , "SECOND HALF OF 16" , "not healthy" , "Rotten, evil" , "Mouldy, off" .)