Deal with most important territory (6)
I believe the answer is:
domain
'territory' is the definition.
(I've seen this before)
'deal with most important' is the wordplay.
'deal with' becomes 'do'.
'most important' becomes 'main' (I've seen this before).
'do'+'main'='DOMAIN'
(Other definitions for domain that I've seen before include "Territory over which rule or control is exercised" , "estate" , "Area under someone's control, field of activity" , "Special region, area of expertise" , "A district sphere of influence" .)