It's a measure of land, dearest heart! (3)
I believe the answer is:
are
'it's a measure of land' is the definition.
(I've seen this before)
'dearest heart' is the wordplay.
'heart' means to look at the middle letters.
The centre of 'dearest' is 'ARE'.
(Other definitions for are that I've seen before include "Era (anag.)" , "(We/you/they) exist" , "live" , "Unit of land measure equal to 100 square metres" , "(We/they/you) exist" .)