Idleness during battle (8)
I believe the answer is:
inaction
'idleness' is the definition.
(thesaurus)
'during battle' is the wordplay.
'during' becomes 'in' (I've seen this in other clues).
'battle' becomes 'action' (military action).
'in'+'action'='INACTION'
(Other definitions for inaction that I've seen before include "Immobility" , "doing nothing whatsoever" , "Inertia" , "Lack of anything being done" , "Dormancy" .)