Mark in revolutionary breeding ground for spooks? (4)

I believe the answer is:

note

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

'mark' is the definition.
(mark can mean to note or remember)

I don't understand how the remainder of the clue works.

(Other definitions for note that I've seen before include "Observe in Eton" , "Jotting" , "Observe - musical symbol" , "Musical tone" , "Sound, record" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app