Important aborigine not one to accommodate Jack (5)
I believe the answer is:
major
'important' is the definition.
(major can mean of great importance)
'aborigine not one to accommodate jack' is the wordplay.
'aborigine' becomes 'Maori' (the Maori are aboriginal people of New Zealand).
'not' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'to accommodate' means one lot of letters goes inside another.
'jack' becomes 'J' (playing card abbreviation).
'maori' with 'i' taken out is 'maor'.
'maor' placed around 'j' is 'MAJOR'.
(Other definitions for major that I've seen before include "Chief - officer" , "Word to describe key" , "Uppermost" , "Of great importance" , "Significant; rank" .)