Malefactor conceals piece of information (4)
I believe the answer is:
fact
'piece of information' is the definition.
(I've seen this in another clue)
'malefactor conceals' is the wordplay.
'conceals' indicates the answer is hidden within the clue.
'FACT' is hidden within 'malefactor'.
(Other definitions for fact that I've seen before include "Something known to be true" , "Statement of verifiable information" , "Fait accompli" , "It's certainly not fiction" , "True statement" .)