Judas Priest's last run -- top-notch nonsense, on reflection (7)
I believe the answer is:
traitor
'judas' is the definition.
(I've seen this before)
'priest's last run top-notch nonsense on reflection' is the wordplay.
'last' suggests the final letters.
'run' becomes 'r' (cricket abbreviation).
'top notch' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'on reflection' shows that the letters should be reversed in order.
The final letter of 'priest' is 't'.
'rot' back-to-front is 'tor'.
't'+'r'+'ai'+'tor'='TRAITOR'
(Other definitions for traitor that I've seen before include "Disloyal person" , "Unreliable figure" , "Turncoat" , "Betrayer, deceiver" , "Backstabber" .)