Data is not proven under trial, for starters (5)
I believe the answer is:
input
'data' is the definition.
(thesaurus)
'is not proven under trial for starters' is the wordplay.
'for starters' suggests taking the first letters.
The first letters of 'is not proven under trial' is 'INPUT'.
(Other definitions for input that I've seen before include "Data entry" , "Signal fed to equipment" , "Simple, without artifice" , "Data contributed" , "Signal feeding a circuit" .)