This is top secret

[Winwin2011]

I read the following sentence from NTC's Dictionary of American English Expression:

Instructions about keeping a secret

This is top secret.

Secret is a countable noun. Why doesn't the author use "a' before "top secret."?

Thanks.

 

[Rover_KE]

'Secret' is an adjective here.

Rover

 

[SoothingDave]

"Top secret" is a classification.

 

[emsr2d2]

Read more about those classifications here.

 

[5jj]

When I wore a uniform, we knew the classifications as TSCRU, pronounced 'Tea-screw'.

 

[emsr2d2]

When I wore a uniform, we knew the classifications as TSCRU, pronounced 'Tea-screw'.

Thank you! I knew there was a mnemonic for it when I first joined the Civil Service but it had been lost in the mists of time. I can't believe that's nowhere to be found on Google.