The most common and most likely answer to this crossword clue is the 7 letter word HONESTY
I've seen this crossword clue in The Sunday Telegraph
H
O
N
E
S
T
Y
More clues leading to the result HONESTY
- Improve farm accommodation? It’s the best policy
- Truly, not large plant
- Sharpen pen, being blunt perhaps
- Saint punching angel, the recommended policy?
- Perfect place for the swine, in fairness
- Virtue that’s cultivated
- Get keen on messy place coming clean
- Optimal policy, darling, when receiving Valentine, say
- Frankness to rectify bluntness of enclosure
- A virtue that’s easily cultivated
Buddy explains!

The straight part
We know that "Optimal policy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case optimal policy = honesty.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 HONESTY mean?
HONESTY noun- The act, quality, or condition of being honest.
- Honor; decency, propriety.
- Chastity.
- Any of various crucifers in the genus Lunaria, several of which are grown as ornamentals, particularly Lunaria annua.