Arrest spies, we're told (5)
I believe the answer is:
seize
'arrest' is the definition.
(thesaurus)
'spies we're told' is the wordplay.
'spies' becomes 'sees'.
'we're told' shows a homophone (sound like).
'sees' is a homophone of 'SEIZE'.
(Other definitions for seize that I've seen before include "Grasp forcibly" , "appropriate" , "Grab by force" , "Take possession" , "Impound" .)