World War I battle certainly captivates pair (5)
I believe the answer is:
ypres
'world war i battle' is the definition.
(1914 battle in World War One)
'certainly captivates pair' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'captivates' means one lot of letters goes inside another.
'pair' becomes 'pr' (abbreviation).
'yes' enclosing 'pr' is 'YPRES'.
(Other definitions for ypres that I've seen before include "Scene of action" , "WWI battle site" , "WW1 battle town" , "WW1 battle location" , "WW1 battle site" .)