Guess I'm in property (8)
I believe the answer is:
estimate
'guess' is the definition.
(I know that guess can be written as estimate)
'i'm in property' is the wordplay.
'in' means one lot of letters goes inside another.
'property' becomes 'estate' (estate is a kind of property).
'im' going inside 'estate' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "damage" , "Meatiest (anag) - figure" , "Calculate roughly" , "Reckon, judge" , "Judge the approximate value" .)