A number, about a thousand, in battle (5)
I believe the answer is:
somme
'battle' is the definition.
(Somme is an example)
'a number about a thousand' is the wordplay.
'a number' becomes 'some' (I have seen 'Undefined number ' mean 'some' so perhaps 'number' could also mean 'some').
'about' indicates putting letters inside.
'a thousand' becomes 'm' ('M' can be a synonym of 'thousand').
'some' going around 'm' is 'SOMME'.
'in' is the link.
(Other definitions for somme that I've seen before include "World War one battle where great losses were incurred" , "Battle site in France" , "runner" , "Scene of World War One battles" , "French river at Amiens" .)