The most common and most likely answer to this crossword clue is the 10 letter word LEGITIMATE
I've seen this crossword clue in The Daily Telegraph
L
E
G
I
T
I
M
A
T
E
More clues leading to the result LEGITIMATE
- Walk with chum around lunchtime, OK?
- Weird game title I authorised
- Run away with one friend to fair
- Run off to get married? Outside Italy that’s allowed
- Board move finally following split on independent permit
- I get meal reinstated, maintaining it’s kosher
- It’s lawful to run away with a friend
- Meal — I get it cooked, OK?
- True friend second to go after support
- Authorised to run away with current partner
Buddy explains!

The straight part
We know that "OK" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ok = legitimate.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 LEGITIMATE mean?
LEGITIMATE noun- A person born to a legally married couple.
- To make legitimate, lawful, or valid; especially, to put in the position or state of a legitimate person before the law, by legal means.
- In accordance with the law or established legal forms and requirements.
- Conforming to known principles, or established or accepted rules or standards; valid.
- Authentic, real, genuine.
- Lawfully begotten, i.e., born to a legally married couple.
- Relating to hereditary rights.