The most common and most likely answer to this crossword clue is the 8 letter word FORBEARS
I've seen this crossword clue in The Daily Telegraph
F
O
R
B
E
A
R
S
Buddy explains!

The straight part
We know that "Keeps in check" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keeps in check = forbears.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.
What does FORBEARS mean?
FORBEARS verb- To keep away from; to avoid; to abstain from.
- To refrain from proceeding; to pause; to delay.
- To refuse; to decline; to withsay; to unheed.
- To control oneself when provoked.
- An ancestor.