Hydrate ground where copper may be found (3,4)
I believe the answer is:
the yard
'where copper may be found' is the definition.
(Scotland Yard, HQ of the Metropolitan Police)
'hydrate ground' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'hydrate' is an anagram of 'THE YARD'.
(Other definitions for the yard that I've seen before include "Met base" , "London Metropolitan Police headquarters" .)