Million taken in an unspecified battle (5)
I believe the answer is:
somme
'battle' is the definition.
(Battle of the Somme in WW1)
'million taken in an unspecified' is the wordplay.
'million' becomes 'm' (abbreviation).
'taken in' is an insertion indicator.
'an unspecified' becomes 'some' (some unspecified thing).
'm' going inside 'some' is 'SOMME'.
(Other definitions for somme that I've seen before include "bloody fight" , "Move in clumsy bounds" , "French river at Amiens" , "1916 battleground" , "French river known from First World War" .)