Replace the acre with a metric measurement (7)
I believe the answer is:
'a metric measurement' is the definition.
(hectare is a metric unit)
'replace the acre' is the wordplay.
'replace' is an anagram indicator (I've seen 'replacing' mean this (placing again)).
'theacre' with letters rearranged gives 'HECTARE'.
'with' acts as a link.
(Other definitions for hectare that I've seen before include "Metric land area" , "Metric unit of area" , "10,000 squaer metres" , "Extent" , "10,000 square metres" .)