Traitor defended by Arthur Atkinson (3)
I believe the answer is:
rat
'traitor' is the definition.
(I've seen this in another clue)
'defended by arthur atkinson' is the wordplay.
'defended by' indicates the answer is hidden within the clue.
'RAT' is hidden within 'arthur atkinson'.
(Other definitions for rat that I've seen before include "Inform" , "Betray like a rodent" , "one who deserts the sinking ship?" , "one escaping doomed vessel?" , "Desert - rodent" .)