The most common and most likely answer to this crossword clue is the 6 plus 5 letter word TROJAN HORSE
I've seen this crossword clue in The Daily Telegraph
T
R
O
J
A
N
H
O
R
S
E
More clues leading to the result TROJAN HORSE
- Present ploy at last decisive in long-term investment
- Hard worker on heroin a danger on computer
- Marshal or others checking M for stratagem
- Hector, perhaps, and mount threat to computer
- The Enemy Within: John, a vagrant, enters resort for bombing
- Computer virus — one that works hard
- Wooden animal, a battle-winner?
- Criminal to John’s rear is old con
- Present method by which company infiltrated the City
Buddy explains!
The straight part
We know that "is old con" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case is old con = trojan horse.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.