Rearrange the parts of the sentence in correct order.
Seven decades ago,
P-Orwell wrote a clutch
Q-for the post-World War II British journal Polemic
R-of essays

Let's analyze the syntax:
The sentence opens with a time marker: "Seven decades ago,".

This should be followed by a subject and a verb, which is found in P: "Orwell wrote a clutch".

The word "clutch" (meaning a group or collection) must be followed by "of" to define what it contains, which leads directly to R: "of essays".

Lastly, Q details where these essays were written for: "for the post-World War II British journal Polemic".

Thus, the sequence is P → R → Q.