Half of Surrey contains key land (6)
I believe the answer is:
realty
'land' is the definition.
(I know that land can be written as realty)
'half of surrey contains key' is the wordplay.
'half of' indicates taking half.
'contains' means one lot of letters goes inside another.
'key' becomes 'alt' (the Alt keyboard button).
'surrey' halved is 'rey'.
'rey' placed around 'alt' is 'REALTY'.
(Other definitions for realty that I've seen before include "Immovable property" , "American real estate" , "Property legally" , "Property ownership in the US" , "Lyrate (anag.)" .)