Urgent knocking (8)

I believe the answer is:

critical

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

'urgent' is the definition.
(I've seen this before)

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

(Other definitions for critical that I've seen before include "Captious, carping or crucial" , "Censorious - to a dangerous degree" , "At crucial stage, or being disparaging" , "of such crucial importance" , "Urgent - fault-finding" .)

I've seen this clue in The Times and The Sunday Times.

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