Advertising stage around fishy location (12)
I believe the answer is:
billingsgate
'fishy location' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'advertising stage around' is the wordplay.
'advertising' becomes 'billing' (I can't justify this - if you can you should give a lot more credence to this answer).
'around' indicates anagramming the letters.
'stage' with letters rearranged gives 'sgate'.
'billing'+'sgate'='BILLINGSGATE'
Can you help me to learn more?
(Other definitions for billingsgate that I've seen before include "Swearing" , "London fish market" .)