The rational approach is to split estate (7)
I believe the answer is:
realism
'the rational approach' is the definition.
'realism' can be an answer for 'approach' (I have seen 'Practical approach' mean 'realism' so perhaps 'approach' could also mean 'realism'). I am not certain of the 'the rational' bit.
'is to split estate' is the wordplay.
'to split' is an insertion indicator (some letters split a word into two).
'estate' becomes 'realm' (I am not sure about this - if you are sure you should believe this answer much more).
'is' placed within 'realm' is 'REALISM'.
Can you help me to learn more?
(Other definitions for realism that I've seen before include "The representation of things as they are" , "Literal style in art" , "Representation in an accurate and true to life way" , "Actuality" , "Practicality of mailers" .)