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' indicates one should take 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' in reverse letter order 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" .)