Backing second new leader in case (8)
I believe the answer is:
sanction
'backing' is the definition.
(thesaurus)
'second new leader in case' is the wordplay.
'second' becomes 's' (common abbreviation).
'leader' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'case' becomes 'action' ('action' can be a synonym of 'case'**).
The initial letter of 'new' is 'n'.
'n' placed inside 'action' is 'anction'.
's'+'anction'='SANCTION'
(Other definitions for sanction that I've seen before include "Official permission or authority" , "Authorisation" , "Official approval (for an action)" , "Support" , "Confirm" .)