Metric area the acre could become (7)
I believe the answer is:
hectare
'metric area' is the definition.
(I've seen this before)
'the acre could become' is the wordplay.
'could become' indicates anagramming the letters.
'the'+'acre'='theacre'
'theacre' is an anagram of 'HECTARE'.
(Other definitions for hectare that I've seen before include "Extent" , "Metric land area" , "The race (anag) - unit of area" , "Metric unit of area" , "Teacher of metric land measure" .)