Meaning: of or characteristic of the activities of spies and undercover agents.

Example:
Do we need to go for cloak-and-dagger way? Why can't we just meet in a cafe like everyone else.

Meaning: a strong and clear request for people to do something.

Example:
The social leader's clarion call for more donations has given rise to a prompt response.

Meaning: not brave.

Example:
These chicken hearted bosses always seem to give in at the first sign of a strike.

Meaning: the particular time when any crucial act supposed to take place.

Example:
Right before the dawn, the British military had been waiting for zero hour to start the operation against terrorist in Afghanistan.