Show to be true
'show to be true' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for prove that I've seen before include "Show to be true" , "Show by evidence" , "Verify" , "Demonstrate by evidence" , "Demonstrate to be true" .)
(Other definitions for bear out that I've seen before include "Confirm (something)" , "Confirm - or carry away" , "Corroborate, support with evidence" , "Confirm - or carry from house" .)