Amount of land that's about to rise and fall (4)
I believe the answer is:
acre
'amount of land' is the definition.
(I've seen this before)
'about to rise and fall' is the wordplay.
'about' becomes 're' (regarding).
'to' says to put letters next to each other (I've seen this in other clues).
'rise and fall' becomes 'ac' (I am not sure about this - if you are sure you should believe this answer much more).
're' after 'ac' is 'ACRE'.
'that's' is the link.
(Other definitions for acre that I've seen before include "Crusaders battlefield, now in Israel" , "in Western Brazil" , "Muslims here besieged" , "Persistent tendency" , "Imperial unit of area" .)