Poster and sign broadcast entrance where fish sold (12)
I believe the answer is:
billingsgate
'where fish sold' is the definition.
(Billingsgate Fish Market in London)
'poster and sign broadcast entrance' is the wordplay.
'poster' becomes 'bill' ('bill' can be a synonym of 'poster'**).
'and' means one lot of letters go next to another.
'broadcast' indicates anagramming the letters.
'entrance' becomes 'gate' (I've seen this before).
'sign' with letters rearranged gives 'ings'.
'bill'+'ings'+'gate'='BILLINGSGATE'
(Other definitions for billingsgate that I've seen before include "Swearing" , "London fish market" .)