Window on opening of fire escape (6)
I believe the answer is:
flight
'escape' is the definition.
(I know that escape can be written as flight)
'window on opening of fire' is the wordplay.
'window' becomes 'light' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'opening of' suggests taking the first letters.
The first letter of 'fire' is 'f'.
'light' put after 'f' is 'FLIGHT'.
(Other definitions for flight that I've seen before include "Air transport - running away" , "the vanquished may take?" , "Exodus" , "A series of stairs" , "Series of steps" .)