Confirm, back up (11)
I believe the answer is:
corroborate
This clue is a double definition.
'confirm' is the first definition.
(corroborating is a kind of confirming)
'back up' is the second definition.
(thesaurus)
(Other definitions for corroborate that I've seen before include "Bear out" , "Back up (a statement)" , "Confirm (a statement)" , "Confirm - support" .)