Maintain agreement must have right to replace name (6)
I believe the answer is:
assert
'maintain' is the definition.
(I know that maintain can be written as assert)
'agreement must have right to replace name' is the wordplay.
I cannot really understand how this works, but
'right' could be 'rt' (abbreviation as in Rt Hon- the Right Honourable) and 'rt' is located in the answer.
This explanation may well be incorrect...
(Other definitions for assert that I've seen before include "State confidently" , "Maintain formally" , "Declare, state vigorously" , "protest" , "Swear" .)