The most common and most likely answer to this crossword clue is the 9 plus 5 letter word WATERGATE HOUSE
I've seen this crossword clue in The Daily Telegraph
W
A
T
E
R
G
A
T
E
H
O
U
S
E
Buddy explains!
The straight part
We know that "once spies were here" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case once spies were here = watergate house.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.