Trained spies prove to be overpowering (10)
I believe the answer is:
oppressive
'be overpowering' is the definition.
(thesaurus)
'trained spies prove' is the wordplay.
'trained' indicates an anagram (to train can mean to grow something into a shape).
'spies'+'prove'='spiesprove'
'spiesprove' with letters rearranged gives 'OPPRESSIVE'.
'to' is the link.
(Other definitions for oppressive that I've seen before include "cruel" , "overpowering!" , "Tyrannical" , "Harsh, authoritarian" , "Sultry" .)