Optimal policy they arranged with son (7)
I believe the answer is:
honesty
'optimal policy' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'they arranged with son' is the wordplay.
'arranged with' is an anagram indicator (letters rearranged).
'they' mixed up with 'son' can make 'HONESTY'.
Can you help me to learn more?
(Other definitions for honesty that I've seen before include "that's easily cultivated" , "Straightforwardness" , "Uprightness" , "Plant; truthfulness" , "It's said to be the best policy" .)