Corroborate or substantiate (7)
I believe the answer is:
confirm
I believe this clue is a double definition.
'corroborate' is the first definition.
(I know that corroborate can be written as confirm)
'substantiate' is the second definition.
(I know that substantiate can be written as confirm)
(Other definitions for confirm that I've seen before include "Establish the truth of" , "Strengthen" , "Philosophy of life" , "Corroborate" , "Make (provisional arrangement) definite" .)