Contentious libel forced company to sack existing leaders (9)
I believe the answer is:
bellicose
'contentious' is the definition.
(thesaurus)
'libel forced company to sack existing leaders' is the wordplay.
'forced' indicates anagramming the letters.
'company' becomes 'co' (abbreviation).
'to' means one lot of letters go next to another.
'leaders' suggests taking the first letters.
The first letters of 'sack existing' is 'se'.
'libel' anagrammed gives 'belli'.
'belli'+'co'+'se'='BELLICOSE'
(Other definitions for bellicose that I've seen before include "Aggressive and willing to fight" , "Pugnacious, eager to fight" , "Combative" , "Aggressive, looking for a fight" , "Jingoistic" .)