The most common and most likely answer to this crossword clue is the 9 letter word SIGNATURE
I've seen this crossword clue in The Daily Telegraph
S
I
G
N
A
T
U
R
E
More clues leading to the result SIGNATURE
- Crumbling ruins get a form of identification
- Certain to include bizarre giant identifying mark
- Name certain to describe one little pest
- Back soldiers with the wild distinctive mark
- Endorsement is rejected by Midge Ure
- Endorsement for Sicilian leaders’ good temperament
- Tune that can make a singer’s name
- As Turing deciphered Enigma, initially used to identify someone
- Identifying pattern that’s bound to squash one irritant
- Uncontrollable urge isn’t a mark of identification
Buddy explains!

The straight part
We know that "mark of identification" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mark of identification = signature.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 SIGNATURE mean?
SIGNATURE noun- A person's name, written by that person, used as identification or to signify approval of accompanying material, such as a legal contract.
- An act of signing one's name; an act of producing a signature.
- The part of a doctor’s prescription containing directions for the patient.
- Signs on the stave indicating key and tempo, composed of the key signature and the time signature.
- A group of four (or a multiple of four) sheets printed such that, when folded, they become a section of a book.
- A pattern used for matching the identity of a virus, the parameter types of a method, etc.
- Data attached to a message that guarantees that the message originated from its claimed source.
- A mark or sign of implication.
- A dish that is characteristic of a particular chef.
- A tuple specifying the sign of coefficients in any diagonal form of a quadratic form.
- A resemblance between the external character of a disease and those of some physical agent, for instance, that existing between the red skin of scarlet fever and a red cloth; supposed to indicate this agent in the treatment of the disease.
- Text (or images, etc.) appended to a user's emails, newsgroup posts, forum posts, etc. as a way of adding a personal touch. Email signatures often include extended contact information. Forum signatures often serve as a way for a user to express themselves (song lyrics, art, etc.).
- Distinctive, characteristic, indicative of identity.