Small piece of bread (5)
I believe the answer is:
crumb
'small piece of bread' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for crumb that I've seen before include "Very small quantity of something, especially food" , "Small piece of bread (or comfort!)" , "A tiny bit of bread" , "Dry morsel" , "Just a little bit" .)