Fleeing battle (had a fill of killing essentially) (6)
I believe the answer is:
flight
'fleeing' is the definition.
(I've seen this before)
'battle had a fill of killing essentially' is the wordplay.
'battle' becomes 'fight' (synonyms).
'had a fill of' is an insertion indicator (some letters fill others).
'essentially' says to take the centre.
The middle of 'killing' is 'l'.
'fight' going around 'l' is 'FLIGHT'.
(Other definitions for flight that I've seen before include "the vanquished may take?" , "The feathers on a dart or arrow" , "Air transport - running away" , "Air force unit; running away" , "Stairs; escape" .)