This corrupts data month after rot reprocessed (6)
I believe the answer is:
trojan
'this corrupts data' is the definition.
(as in a trojan horse computer program)
'month after rot reprocessed' is the wordplay.
'month' becomes 'jan' (abbreviation for January).
'after' means one lot of letters go next to another.
'reprocessed' indicates an anagram (letters processed or transformed).
'rot' is an anagram of 'tro'.
'jan' after 'tro' is 'TROJAN'.
(Other definitions for trojan that I've seen before include "Native of Troy" , "Native like Helen" , "This horse once concealed Greek warriors bent on breaking a siege" , "Computer threat" , "Paris, say" .)