The most common and most likely answer to this crossword clue is the 7 letter word CASSIUS
I've seen this crossword clue in The Guardian
C
A
S
S
I
U
S
More clues leading to the result CASSIUS
- Conservative idiot is hanging around university, a plotter
- One who conspired with accountants originally installed in S America
- Clay, perhaps, brought by small spades into college
- Cambridge college admitting sons of old assassin
- Person behind conspiracy to kill Caesar and an old Pope held by secret police? Quite the contrary
Buddy explains!
The straight part
We know that "old assassin" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old assassin = cassius.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.