The most common and most likely answer to this crossword clue is the 5 letter word LABEL
I've seen this crossword clue in The Daily Telegraph
L
A
B
E
L
More clues leading to the result LABEL
- Tag left next on murderee
- Tag lines Lincoln introduced
- Handle dog well, but only intermittently
- Sheila Bellingham’s trademark
- Left Lincoln with second-class ticket
- Identify murder victim found beneath lake
- Means of identification left on murder victim
- Tag left on early murder victim
- What may be used to identify body of old murder victim?
- Calumny I ignored, retaining a name
Buddy explains!

The straight part
We know that "attachment to the Christmas parcel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case attachment to the christmas parcel = label.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 LABEL mean?
LABEL noun- A small ticket or sign giving information about something to which it is attached or intended to be attached.
- A name given to something or someone to categorise them as part of a particular social group.
- A company that sells records.
- A user-defined alias for a numerical designation, the reverse of an enumeration.
- A named place in source code that can be jumped to using a GOTO or equivalent construct.
- A charge resembling the strap crossing the horse’s chest from which pendants are hung.
- A tassel.
- A piece of writing added to something, such as a codicil appended to a will.
- A brass rule with sights, formerly used with a circumferentor to take altitudes.
- The projecting moulding by the sides, and over the tops, of openings in mediaeval architecture.
- In mediaeval art, the representation of a band or scroll containing an inscription.
- A non-interactive control or widget displaying text, often used to describe the purpose of another control.
- To put a label (a ticket or sign) on (something).
- (ditransitive) To give a label to (someone or something) in order to categorise that person or thing.
- To replace specific atoms by their isotope in order to track the presence or movement of this isotope through a reaction, metabolic pathway or cell.
- To add a detectable substance, either transiently or permanently, to a biological substance in order to track the presence of the label-substance combination either in situ or in vitro