Second head of security upset secret service (6)
I believe the answer is:
mossad
'secret service' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'second head of security upset' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'head of' says to take the initial letters.
'upset' becomes 'sad' (I've seen this in another clue).
The first letter of 'security' is 's'.
'mo'+'s'+'sad'='MOSSAD'
Can you help me to learn more?
(Other definitions for mossad that I've seen before include "Agents" , "Israeli intelligence agency" , "Israeli intelligence service" , "Foreign intelligence agency" , "Israeli secret service" .)