Without supposedly responsible adults driving train, pair sent off the rails (2,4,8)
I believe the answer is:
in loco parentis
'without' is the definition.
Both the answer and definition are adverbs.
Perhaps you can see a link between them that I can't see?
'supposedly responsible adults driving train pair sent off the rails' is the wordplay.
I cannot quite see how this works, but
'train' could be 'loco' (loco is short for locomotive) and 'loco' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for in loco parentis that I've seen before include "old man?" , "Looking after wards, perhaps" , "As a teacher may be" , "with kids under his wing?" , "instead of the parents" .)