Several crosses mark battlefield (5)
I believe the answer is:
somme
'battlefield' is the definition.
(I've seen this before)
'several crosses mark' is the wordplay.
'several' becomes 'some' (similar in meaning).
'crosses' is an insertion indicator.
'mark' becomes 'm'.
'some' going around 'm' is 'SOMME'.
(Other definitions for somme that I've seen before include "French river known from First World War" , "French 2 down" , "WW1 battlefield" , "Major Great War battle" , "French river at Amiens" .)