The whole of that corner of the crossword would fall into place!
Is STICK a word for polo mallet?
RUSTIC allows LOCK GATE, allows UPHILL for Jack, and would give me for the following clue:
For Americans, quite the best book (6)
P - U - -.
For the dukes clue, I have ENNOBLING, which would add a B, so P - U - B.
Looks like PLUMB.