Reckon I'm splitting the estate (8)
I believe the answer is:
estimate
'reckon' is the definition.
(estimating is a kind of reckoning)
'i'm splitting the estate' is the wordplay.
'splitting' is an insertion indicator.
'im' going into 'estate' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Judge the approximate cost of" , "Make an approximate assessment" , "Make an educated guess" , "Reckon value" , "rough judgment" .)