Observes energy shown by secret agents (6)
I believe the answer is:
espies
'observes' is the definition.
'energy shown by secret agents' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'shown by' says to put letters next to each other.
'secret agents' becomes 'spies' (spy is a kind of secret agent).
'e'+'spies'='ESPIES'
(Other definitions for espies that I've seen before include "Spots" , "Sees (archaic)" , "Catches sight of" .)