A piece of baked pottery, whether whole or a fragment (5)
I believe the answer is:
testa
'a piece' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for testa that I've seen before include "Hard shell" , "Protective covering" , "hard case" , "Botanical term" , "protective coat" .)