The most common and most likely answer to this crossword clue is the 8 letter word TESTABLE
I've seen this crossword clue in The Sunday Telegraph
T
E
S
T
A
B
L
E
Buddy explains!
The straight part
We know that "OK for checking" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ok for checking = testable.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 TESTABLE mean?
TESTABLE adjective- Susceptible to being tested.
- With respect to the scientific method, capable of being proven true or false.
- Capable of being devised, or given by will.