The most common and most likely answer to this crossword clue is the 9 letter word WELL-USED
I've seen this crossword clue in Financial Times
W
E
L
L
-
U
S
E
D
Buddy explains!
The straight part
We know that "being in common operation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case being in common operation = well-used.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 WELL-USED mean?
WELL-USED adjective- Used a great deal; having seen a lot of use.