Sharpen pen to a measure of straightness (7)
I believe the answer is:
honesty
'measure of straightness' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'sharpen pen' is the wordplay.
'sharpen' becomes 'hone' (honing is a kind of sharpening).
'pen' becomes 'sty' (both can mean an animal enclosure).
'hone'+'sty'='HONESTY'
'to a' is the link.
Can you help me to learn more?
(Other definitions for honesty that I've seen before include "Probity; plant" , "It's said to be the best policy" , "Uprightness" , "Candour" , "Plant; sincerity" .)