The shrewdness of men with some faculties (6)
I believe the answer is:
acumen
'the shrewdness' is the definition.
(acumen can mean shrewdness or good judgment)
'men with some faculties' is the wordplay.
'with' is a charade indicator (letters next to each other).
'some faculties' becomes 'acu' (I can't explain this - if you can you should give a lot more credence to this answer).
'men' after 'acu' is 'ACUMEN'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for acumen that I've seen before include "Astuteness, penetration" , "Forming judgement" , "Mental keenness" , "intuition" , "Ability to judge quickly and well" .)