Seafood — not as much being brought up under a lot of mud (6)
I believe the answer is:
mussel
'seafood' is the definition.
(I know that mussel is a type of seafood)
'not as much being brought up under a lot of mud' is the wordplay.
'not as much' becomes 'less' (similar in meaning).
'being brought up' is a reversal indicator (in a down clue, letters go up).
'under' says to put letters next to each other (below, in a down clue).
'a lot of' means to remove the last letter (a lot of but not all of the word).
'less' in reverse letter order is 'ssel'.
'mud' with its final letter taken away is 'mu'.
'ssel' after 'mu' is 'MUSSEL'.
(Other definitions for mussel that I've seen before include "Black marine bivalves" , "Edible marine mollusc sold by Molly Malone - not the cockle" , "Common bivalve shellfish, anchors to rocks" , "Edible shellfish" , "Pearl may come from it" .)