Maintain right to divide valuable item (6)
I believe the answer is:
assert
'maintain' is the definition.
('assert' can be a synonym of 'maintain')
'right to divide valuable item' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to divide' indicates putting letters inside.
'valuable item' becomes 'asset' (I've seen this before).
'r' going inside 'asset' is 'ASSERT'.
(Other definitions for assert that I've seen before include "Declare, claim" , "State firmly or vigorously" , "protest" , "State firmly, aver" , "Aver, maintain" .)