Advertising on small portal to market (12)
I believe the answer is:
billingsgate
'market' is the definition.
(London market)
'advertising on small portal' is the wordplay.
'advertising on small' becomes 'billings' (I can't justify this - if you can you should believe this answer much more).
'portal' becomes 'gate' (I've seen this in another clue).
'billings'+'gate'='BILLINGSGATE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for billingsgate that I've seen before include "London fish market" , "Swearing" .)