Possible Clues:

• Agent that undermines from within
• Criminal to John's rear is old con
• Hard worker overcomes drug trap
• Destructive computer virus
• Investment aid produced by hard worker and major consumer
• Computer problem that brought an end to investment
• Something introduced to defeat from within
• One working hard to overcome drug, seeing trap
• A trap to be wary of (a wooden one?)
• Hard worker on heroin's admitted losing the war