Reckon I'm in landed property (8)
I believe the answer is:
estimate
'reckon' is the definition.
(I know that reckon can be written as estimate)
'i'm in landed property' is the wordplay.
'in' indicates putting letters inside.
'landed property' becomes 'estate' (I've seen this in another clue).
'im' placed within 'estate' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Guess the cost" , "Tease Tim (anag.)" , "Meatiest (anag) - figure" , "Make an educated guess" , "Calculate in approximate way" .)