The most common and most likely answer to this crossword clue is the 5 letter word DIZZY
I've seen this crossword clue in Financial Times
D
I
Z
Z
Y
Buddy explains!

The straight part
We know that "Light-headed/PM of yesteryear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case light-headed/pm of yesteryear = dizzy.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DIZZY mean?
DIZZY verb- To make dizzy, to bewilder.
- Having a sensation of whirling, with a tendency to fall; giddy; feeling unbalanced or lightheaded.
- Producing giddiness.
- Empty-headed, scatterbrained or frivolous; ditzy.