The most common and most likely answer to this crossword clue is the 14 letter word TOILET-TRAINED
I've seen this crossword clue in The Guardian
T
O
I
L
E
T
-
T
R
A
I
N
E
D
Buddy explains!

The straight part
We know that "able to use bathroom" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case able to use bathroom = toilet-trained.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-TRAINED mean?
TOILET-TRAINED verb- To train (a child) to use the toilet.
- To train (a domestic animal) to defecate and urinate in certain designated places.