The most common and most likely answer to this crossword clue is the 12 letter word PRESCRIPTION
I've seen this crossword clue in The Daily Telegraph
P
R
E
S
C
R
I
P
T
I
O
N
More clues leading to the result PRESCRIPTION
Buddy explains!
The straight part
We know that "instruction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instruction = prescription.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 PRESCRIPTION mean?
PRESCRIPTION noun- The act of prescribing a rule, law, etc..
- Also called extinctive prescription or liberative prescription. A time period within which a right must be exercised, otherwise it will be extinguished.
- Also called acquisitive prescription. A time period after which a person who has, in the role of an owner, uninterruptedly, peacefully, and publicly possessed another's property acquires the property. The described process is known as acquisition by prescription and adverse possession.
- A written order, as by a physician or nurse practitioner, for the administration of a medicine or other intervention. See also scrip.
- The prescription medicine or intervention so prescribed.
- The formal description of the lens geometry needed for spectacles, etc..
- The act or practice of laying down norms of language usage, as opposed to description, i.e. recording and describing actual usage.
- An instance of a prescriptive pronouncement.
- A plan or procedure to obtain a given end result; a recipe.
- Circumscription; restraint; limitation.
- (of a drug, etc.) only available with a physician or nurse practitioner's written prescription