The most common and most likely answer to this crossword clue is the 2 plus 4 plus 8 letter word IN LOCO PARENTIS
I've seen this crossword clue in The Sunday Telegraph
I
N
L
O
C
O
P
A
R
E
N
T
I
S
More clues leading to the result IN LOCO PARENTIS
- Crazy old man let into home is doing his job?
- Where cop can be seen acting as guardian
- Responsible for an issue raised by others?
- Acting as guardians, arranged oral inspection
- Organised pic with son and relation having guardianship role
- A licit person, no delinquent, given juvenile custody
- Caroline points out law to act as a guardian
- Without supposedly responsible adults driving train, pair sent off the rails
Buddy explains!

The straight part
We know that "Without supposedly responsible adults" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case without supposedly responsible adults = in loco parentis.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.