Establish as valid (12)
(Other definitions for substantiate that I've seen before include "Confirm" , "Prove to be true" , "Prove truth of (charge)" .)
'establish as valid' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for authenticate that I've seen before include "Prove to be genuine" , "Show to be genuine" , "Validate" , "Establish as valid" , "Confirm as genuine" .)