Insect spies circling dog (6)
I believe the answer is:
cicada
'insect' is the definition.
(cicada is a kind of insect)
'spies circling dog' is the wordplay.
'spies' becomes 'CIA' (Central Intelligence Agency).
'circling' indicates putting letters inside (circling can mean going around or containing).
'dog' becomes 'cad' (synonyms).
'cia' going around 'cad' is 'CICADA'.
(Other definitions for cicada that I've seen before include "Large chirping insect" , "Chirpy little thing" , "jumpy creature" , "Insect with a high- pitched drone" , "Droning insect" .)