Units of land area (5)
I believe the answer is:
acres
'units of land area' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acres that I've seen before include "Scare (anag)" , "Imperial units or measures of land" , "The Phoenix Park has 40 of them (5)" , "Cares about these measurements?" , "Land areas" .)