The most common and most likely answer to this crossword clue is the 12 letter word NINE-TO-FIVE
I've seen this crossword clue in The Times
N
I
N
E
-
T
O
-
F
I
V
E
Buddy explains!

The straight part
We know that "routine" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case routine = nine-to-five.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 NINE-TO-FIVE mean?
NINE-TO-FIVE adjective- Of a job, beginning at 9 a.m. and ending at 5 p.m., the usual office hours in many countries.