A soldier, one taking part in WW1 battle (6)
I believe the answer is:
marine
'a soldier' is the definition.
(I've seen this in other clues)
'one taking part in ww1 battle' is the wordplay.
'one' becomes 'i' (Roman numeral).
'taking part in' indicates putting letters inside.
'ww1 battle' becomes 'marne' (First or Second Battle of the Marne in WW1).
'i' inserted into 'marne' is 'MARINE'.
(Other definitions for marine that I've seen before include "Relating to shipping or naval matters" , "Concerned with the sea" , "Airmen (anag.)" , "Relating to the sea" , "Member of the forces" .)