Clue: Burt Lancaster, initially, when prime minister could have met three sisters here (7,5)
Answer: BLASTED HEATH
Explanation: Burt Lancaster, initially, when prime minister could have met three sisters here (7,5)
B(urt) L(ancaster) + AS TED HEATH 
From Act 1, Scene 3 of Macbeth
 
Stay, you imperfect speakers, tell me more: 
By Sinel's death I know I am thane of Glamis; 
But how of Cawdor? the thane of Cawdor lives, 
A prosperous gentleman; and to be king 
Stands not within the prospect of belief, 
No more than to be Cawdor. Say from whence 
You owe this strange intelligence? or why 
Upon this blasted heath you stop our way 
With such prophetic greeting? Speak, I charge you.