Area of land a gang reduced (4)
I believe the answer is:
acre
'area of land' is the definition.
(measurement of land)
'a gang reduced' is the wordplay.
'gang' becomes 'crew' (synonyms).
'reduced' means to remove the last letter.
'crew' with its final letter taken off is 'cre'.
'a'+'cre'='ACRE'
(Other definitions for acre that I've seen before include "Persistent tendency" , "Unit of land area" , "Unit of land measurement" , "Imperial land measure" , "Square measure of land" .)