Bug rogue planted in American agency (6)
I believe the answer is:
cicada
'bug' is the definition.
(cicada is a type of insect)
'rogue planted in american agency' is the wordplay.
'rogue' becomes 'cad' (I've seen this in another clue).
'planted in' means one lot of letters goes inside another.
'american agency' becomes 'CIA' (Central Intelligence Agency).
'cad' inserted within 'cia' is 'CICADA'.
(Other definitions for cicada that I've seen before include "Stridulating insect" , "Droning insect" , "Noisy tropical insect" , "six-footer?" , "Noisy bug" .)