Shellfish turned up whole -- advertise briefly (6)
I believe the answer is:
mussel
'shellfish' is the definition.
(I know that mussel is a type of shellfish)
'turned up whole advertise briefly' is the wordplay.
'turned up' shows that the letters should be reversed in order.
'whole' becomes 'sum' (sum is a kind of whole).
'advertise briefly' becomes 'sel' (I can't justify this - if you can you should believe this answer much more).
'sum' back-to-front is 'mus'.
'mus'+'sel'='MUSSEL'
Can you help me to learn more?
(Other definitions for mussel that I've seen before include "Seafood item" , "Edible bivalve mollusc, common in sea" , "Shellfish sounds like part of the body" , "Edible shellfish" , "Pearl may come from it" .)