Doctor agreed aunt swore (10)
I believe the answer is:
guaranteed
'swore' is the definition.
(I've seen this before)
'doctor agreed aunt' is the wordplay.
'doctor' is an anagram indicator.
'agreed'+'aunt'='agreedaunt'
'agreedaunt' is an anagram of 'GUARANTEED'.
(Other definitions for guaranteed that I've seen before include "protected" , "swore" , "Promised" , "Covered against failure" , "Supported" .)