Piece of bread excellent, taken by a large class (9)
I believe the answer is:
crustacea
'large class' is the definition.
(large class of arthropods)
'piece of bread excellent taken by a' is the wordplay.
'piece of bread' becomes 'crust' (bread has a crust).
'excellent' becomes 'ace' (I've seen this before).
'taken by' is a charade indicator (letters next to each other).
'crust'+'ace'+'a'='CRUSTACEA'
(Other definitions for crustacea that I've seen before include "class of seafood" , "Prawns and lobster" , "crabby group?" , "Collection of hardbacks" , "Group including lobsters, crabs, etc" .)