Unnerve a secret agent (5)

I believe the answer is:

spook

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'unnerve' is the first definition.
(I've seen this in another clue)

'a secret agent' is the second definition.
'spook' can be an answer for 'agent' (I've seen this before). I am not sure about the 'a secret' bit.

Can you help me to learn more?

(Other definitions for spook that I've seen before include "Frighten - haunt - spy" , "Startle - spy" , "Enterprise propels him" , "Ghost - US spy" , "Ghost, spectre" .)

I've seen this clue in The Daily Express.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.