In battle fifty escape (6)
I believe the answer is:
flight
'escape' is the definition.
(I know that escape can be written as flight)
'in battle fifty' is the wordplay.
'in' is an insertion indicator.
'battle' becomes 'fight' (fight is a kind of battle).
'fifty' becomes 'l' (Roman numeral).
'fight' placed around 'l' is 'FLIGHT'.
(Other definitions for flight that I've seen before include "Voyage of an aircraft" , "Series of steps" , "the vanquished may take?" , "Air transport - running away" , "Attachment to missile < part of Air Force" .)