Substantiation (5)
'substantiation' is the definition.
(I know that substantiation can be written as proof)
This is the entire clue.
(Other definitions for proof that I've seen before include "Mathematical argument" , "Positive evidence" , "Testimony" , "Specimen" , "mathematical tour de force" .)
'substantiation' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for evidence that I've seen before include "Witness's testimony" , "Indication" , "Supporting facts" , "authentication" , "grounds" .)