Mark's overcome by a certain World War One battle (5)
I believe the answer is:
somme
'world war one battle' is the definition.
(Battle of the Somme, 1916)
'mark's overcome by a certain' is the wordplay.
'mark' becomes 'm'.
'overcome by' is an insertion indicator.
'a certain' becomes 'some' (eg some person means a certain person).
'm' placed inside 'some' is 'SOMME'.
(Other definitions for somme that I've seen before include "battlesite" , "bloody fight" , "World War one battle where great losses were incurred" , "Battle site in France" , "European river" .)