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I'm stuck on the SW corner. If I could get 21ac, that would be a huge help.

Hints preferable to solutions.

21ac Velocity regulator using gravitational force in preference to negative resistance (8)
I have: ---E---R

All help muchly appreciated.

It's a device used particularly in steam engines. The answer is built up in the order given in the clue.

gravity + 4 letter word for in preference to + 2 letter word for negative + R

Thank you both! Seems so easy when you have it explained. :)