Thus begins battle cry (3)
I believe the answer is:
sob
'cry' is the definition.
(sobbing is a kind of crying)
'thus begins battle' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'begins' suggests taking the first letters.
The initial letter of 'battle' is 'b'.
'so'+'b'='SOB'
(Other definitions for sob that I've seen before include "Cry noisily" , "Weep convulsively" , "Weep uncontrollably" , "Shake with tears" , "Contemptible person in North America briefly" .)