The most common and most likely answer to this crossword clue is the 7 plus 6 letter word ARUNDEL CASTLE
A
R
U
N
D
E
L
C
A
S
T
L
E
Buddy explains!
The straight part
We know that "Keep" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keep = arundel castle.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.