The most common and most likely answer to this crossword clue is the 8 plus 2 plus 5 letter word CREATURE OF HABIT
C
R
E
A
T
U
R
E
O
F
H
A
B
I
T
Buddy explains!
The straight part
We know that "person adhering to routine" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case person adhering to routine = creature of habit.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.