Stand behind aunt and agree changes (9)
I believe the answer is:
guarantee
'stand behind' is the definition.
(thesaurus)
'aunt and agree changes' is the wordplay.
'and' means one lot of letters go next to another.
'changes' indicates an anagram.
'aunt'+'agree'='auntagree'
'auntagree' anagrammed gives 'GUARANTEE'.
(Other definitions for guarantee that I've seen before include "Warranty, pledge" , "Ensure" , "Something ensuring a particular outcome" , "Underwrite; promise with certainty" , "Undertake" .)