Hard worker — malicious software (6)
I believe the answer is:
trojan
I believe this is a double definition.
'hard worker' is the first definition.
(I've seen this before)
'malicious software' is the second definition.
(I know that Trojan is a type of malicious software)
(Other definitions for trojan that I've seen before include "Paris for one" , "Subject of King Priam" , "Dedicated worker" , "Computer threat" , "Of Troy or its people" .)