Clue: Archdeacon attending hospital department getting warning about a couple of veins (5,5)
Answer: VENAE CAVAE
Explanation: Definition: couple of veins
**Ven. **(venerable, address for an "archdeacon") *attending* **A**&**E** (accident and emergency, so "hospital department") *getting* **CAVE** (Latin for "warning") *about* **A** 
*The "venae cavae" are the two major veins which take blood to the heart.