Infallible idiot gets evidence (9)
I believe the answer is:
foolproof
'infallible' is the definition.
(similar in meaning)
'idiot gets evidence' is the wordplay.
'idiot' becomes 'fool' (both can mean a stupid person).
'gets' says to put letters next to each other.
'evidence' becomes 'proof' (evidence proves something).
'fool'+'proof'='FOOLPROOF'
(Other definitions for foolproof that I've seen before include "sure to work" , "tried and tested" , "incapable of going wrong" , "Completely safe" , "Infallible" .)