A few capture the start of military battle (5)
I believe the answer is:
somme
'battle' is the definition.
(Battle of the Somme in WW1)
'a few capture the start of military' is the wordplay.
'a few' becomes 'some' (both can mean a small amount of something).
'capture' indicates putting letters inside.
'the start of' indicates taking the first letters.
The initial letter of 'military' is 'm'.
'some' going around 'm' is 'SOMME'.
(Other definitions for somme that I've seen before include "WWI battle site" , "French river known from First World War" , "French river at Amiens" , "battlesite" , "bloody fight" .)