Sees key agents (6)
I believe the answer is:
espies
'sees' is the definition.
(I've seen this before)
'key agents' is the wordplay.
'key' becomes 'e' (musical key).
'agents' becomes 'spies' (spy is a kind of agent).
'e'+'spies'='ESPIES'
(Other definitions for espies that I've seen before include "Catches sight of" , "Sees (archaic)" , "Spots" .)