Field units (5)
I believe the answer is:
acres
'field units' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acres that I've seen before include "The Phoenix Park has 40 of them (5)" , "One of them has 4840 square yards" , "may burden landed folk" , "Areas of 4840 square yards" , "Cares about the tracts of land" .)