Backing second new leader in battle... (8)
I believe the answer is:
sanction
'backing' is the definition.
(thesaurus)
'second new leader in battle' is the wordplay.
'second' becomes 's' (common abbreviation).
'leader' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'battle' becomes 'action' (military action).
The initial letter of 'new' is 'n'.
'n' going inside 'action' is 'anction'.
's'+'anction'='SANCTION'
(Other definitions for sanction that I've seen before include "Penalty for breaking law" , "Endorsement" , "punitive action" , "Allow; punish" , "measure taken against offending country?" .)