Preposterous Beeb broadcast with nil value (12)
I believe the answer is:
unbelievable
'preposterous' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for unbelievable that I've seen before include "Impossible to credit" , "Incredible" , "Far-fetched" .)