Moved tile 1mm — it fixed window (4,5)
I believe the answer is:
time limit
'window' is the definition.
(window of opportunity in which to do something)
I can't explain the rest of the clue.
(Other definitions for time limit that I've seen before include "within which task must be done" , "maximum duration" , "Deadline" , "Point before which something must be achieved" , "Prescribed term" .)