Dictator managed to infiltrate government department (6)
I believe the answer is:
franco
'dictator' is the definition.
(Spanish dictator Francisco Franco)
'managed to infiltrate government department' is the wordplay.
'managed' becomes 'ran' (I've seen this before).
'to infiltrate' indicates putting letters inside.
'government department' becomes 'FCO' (Foreign and Commonwealth Office).
'ran' put within 'fco' is 'FRANCO'.
(Other definitions for franco that I've seen before include "Last dictator of Spain" , "Former Spanish dictator" , "Former Spanish leader" , "was head of state" , "General . . . . . . was Spain's last dictator" .)