Up to the time when (5)
I believe the answer is:
until
'up to the time when' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for until that I've seen before include "Pending such time as" , "As far as" , "Up to that moment" , "Up to (a point in time)" , "Unlit (anag.)" .)