See revolutionary cycling round Greek island? That's hush-hush (3-6)
I believe the answer is:
top-secret
'hush-hush' is the definition.
(I've seen this before)
'see revolutionary cycling round greek island?' is the wordplay.
I cannot quite understand how this works, but
'see' could be 'c' (the word for the letter, according to Chambers) and 'c' is present in the answer.
'round' could be 'top' (top is a kind of round) and 'top' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'that's' is the link.
Can you help me to learn more?
(Other definitions for top-secret that I've seen before include "Very hush-hush" , "Extremely restricted (information)" , "Highly confidential" , "Highly restricted (information)" , "Erect post (anag)" .)