Small parasites that infest piece of bread (5)
I believe the answer is:
slice
'small parasites that infest piece of bread' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'small parasites' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'parasites' becomes 'lice' (lice are parasitic insects).
's'+'lice'='SLICE'
Can you help me to learn more?
(Other definitions for slice that I've seen before include "share" , "Cut into portions" , "shot in golf that goes astray" , "Wedge" , "Portion (of cake)" .)