The most common and most likely answer to this crossword clue is the 12 letter word UNRESTRAINED
I've seen this crossword clue in The Daily Telegraph
U
N
R
E
S
T
R
A
I
N
E
D
More clues leading to the result UNRESTRAINED
- Rowdy inn in Treasure Island ultimately abandoned
- Without control, public disorder descended
- … strangely ensured it ran wild
- Irrepressible new nurse underwent instruction
- Free nude in arrest, surprisingly
- A French king forced to keep English free
- Ensured train is rescheduled but not properly checked
- Free support before shower in nude, unusually
- Without control, fell after rising
- Rampant disorder precipitated
Buddy explains!

The straight part
We know that "not properly checked" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not properly checked = unrestrained.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 UNRESTRAINED mean?
UNRESTRAINED verb- To free from restraints.
- Immoderate; not restrained or held in check
- Spontaneous, natural and informal; unconstrained
- Not subject to physical restraint.