Supported aunt and agreed changes (10)
I believe the answer is:
guaranteed
'supported' is the definition.
(guaranteeing is a kind of supporting)
'aunt and agreed changes' is the wordplay.
'and' means one lot of letters go next to another (I've seen this in other clues).
'changes' indicates an anagram.
'aunt' after 'agreed' is 'agreedaunt'.
'agreedaunt' with letters rearranged gives 'GUARANTEED'.
(Other definitions for guaranteed that I've seen before include "protected" , "Warranted" , "Covered against failure" , "Ensured" , "swore" .)