Prompt English journalist to probe possible traitor (6)
I believe the answer is:
speedy
'prompt' is the definition.
(I've seen this in another clue)
'english journalist to probe possible traitor' is the wordplay.
'english' becomes 'e' (abbreviation).
'journalist' becomes 'ed' (short for editor).
'to probe' is an insertion indicator (probe can mean to poke or prod into).
'possible traitor' becomes 'spy' (a spy might betray their country).
'e'+'ed'='eed'
'eed' placed into 'spy' is 'SPEEDY'.
(Other definitions for speedy that I've seen before include "in a hurry" , "in a fast car?" , "'Fast, without delay (6)'" , "Rapid, fast" , "Fast moving" .)