The most common and most likely answer to this crossword clue is the 7 plus 3 plus 4 letter word WALKING THE BEAT
I've seen this crossword clue in The Daily Telegraph
W
A
L
K
I
N
G
T
H
E
B
E
A
T
Buddy explains!
The straight part
We know that "PC operating system rarely seen nowadays" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pc operating system rarely seen nowadays = walking the beat.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.