Reckon I am entering property (8)
I believe the answer is:
estimate
'reckon' is the definition.
(estimating is a kind of reckoning)
'i am entering property' is the wordplay.
'i am' becomes 'im'.
'entering' is an insertion indicator.
'property' becomes 'estate' (estate is a kind of property**).
'im' going into 'estate' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Make an educated guess" , "Gauge - approximation" , "Guess the cost" , "rough judgment" , "Make approximate judgement of size or amount" .)