The most common and most likely answer to this crossword clue is the 14 letter word POTTY-TRAINING
I've seen this crossword clue in The Guardian
P
O
T
T
Y
-
T
R
A
I
N
I
N
G
Buddy explains!
The straight part
We know that "instruction in nursery" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instruction in nursery = potty-training.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 POTTY-TRAINING mean?
POTTY-TRAINING noun- The act of weaning a child off of diapers and teaching them how to use a potty. Not usually distinguished from toilet training although generally children do not use an adult-sized toilet right away.