Confine one particle in a hill (7)
I believe the answer is:
impound
'confine' is the definition.
(I know that impound is a more specific form of confine)
'one particle in a hill' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for impound that I've seen before include "Seize and confine (illegal vehicle)" , "Seize and take into legal custody" , "Seize (goods) legally" , "Take property into legal custody" , "appropriate" .)