The most common and most likely answer to this crossword clue is the 15 letter word TOILET-TRAINING
I've seen this crossword clue in The Guardian
T
O
I
L
E
T
-
T
R
A
I
N
I
N
G
Buddy explains!
The straight part
We know that "Tot learning it, one getting potty" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tot learning it, one getting potty = toilet-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 TOILET-TRAINING mean?
TOILET-TRAINING verb- To train (a child) to use the toilet.
- To train (a domestic animal) to defecate and urinate in certain designated places.