Units of land in mostly sacred ground (5)
I believe the answer is:
acres
'units of land' is the definition.
(I've seen this before)
'mostly sacred ground' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'sacred' with its last letter removed is 'sacre'.
'sacre' with letters rearranged gives 'ACRES'.
'in' is the link.
(Other definitions for acres that I've seen before include "Units of land measurement" , "Imperial units or measures of land" , "large area of land" , "Units of area" , "Cares about these measurements?" .)