About 91 yards of a football field (4)
I believe the answer is:
acre
'about 91 yards of a football field' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acre that I've seen before include "Not a lot of land" , "Square measure of land" , "Site of battle" , "Unit of land measurement" , "Care about this bit of land" .)