Assurance in battle-cry (7)
I believe the answer is:
warrant
'assurance' is the definition.
(warrant is a kind of assurance)
'battle-cry' is the wordplay.
'battle' becomes 'war' (I've seen this before).
'cry' becomes 'rant' (I've seen this in other clues).
'war'+'rant'='WARRANT'
'in' is the link.
(Other definitions for warrant that I've seen before include "Call for" , "Guaranty - authorisation to make an arrest" , "Justify; authorisation" , "Merit, justify" , "Guarantee" .)