A pen enclosing about this amount of land (7)
I believe the answer is:
acreage
'this amount of land' is the definition.
(I've seen this before)
'a pen enclosing about' is the wordplay.
'pen' becomes 'cage' (I've seen this before).
'enclosing' is an insertion indicator.
'about' becomes 're' (regarding**).
'cage' going around 're' is 'creage'.
'a'+'creage'='ACREAGE'
(Other definitions for acreage that I've seen before include "Amount of land" , "Number of area measurements" , "Extent of ground" , "Land area" , "Measure of land" .)