A leak in which litres may come out (6)
I believe the answer is:
asleep
'out' is the definition.
(out can mean unconscious or asleep)
'a leak in which litres' is the wordplay.
'leak' becomes 'seep' (I've seen this in another clue).
'in which' indicates putting letters inside.
'litres' becomes 'L' (abbreviation).
'seep' placed around 'l' is 'sleep'.
'a'+'sleep'='ASLEEP'
'may come' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for asleep that I've seen before include "Please be at rest, oddly" , "In the arms of Hypnos" , "(Of a limb) numb" , "Inattentive; numbed" , "Like kippers" .)