The most common and most likely answer to this crossword clue is the 5 letter word VALID
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More clues leading to the result VALID
Buddy explains!

The straight part
We know that "perfectly OK" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case perfectly ok = valid.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 VALID mean?
VALID adjective- Well grounded or justifiable, pertinent.
- Acceptable, proper or correct; in accordance with the rules.
- Related to the current topic, or presented within context, relevant.
- Of a formula or system: such that it evaluates to true regardless of the input values.
- Of an argument: whose conclusion is always true whenever its premises are true.
- Effective.