Ready to fight call that is about function (9)
I believe the answer is:
bellicose
'ready to fight' is the definition.
(bellicose means willing to fight or go to war)
'call that is about function' is the wordplay.
'call' becomes 'bell' (bell can mean a phone call).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'about' means one lot of letters goes inside another.
'function' becomes 'cos' (mathematical function).
'ie' enclosing 'cos' is 'icose'.
'bell'+'icose'='BELLICOSE'
(Other definitions for bellicose that I've seen before include "Belligerent" , "Jingoistic" , "Warlike" , "Combative" , "Aggressive and willing to fight" .)