Fighting people ring about entering office after one (12)

'fighting people' is the definition.
'belligerents' can be an answer for 'people' (belligerent is a kind of person). I am unsure of the 'fighting' bit.

'ring about entering office after one' is the wordplay.
'ring' becomes 'bell' (both can mean to call on the phone).
'about entering office' becomes 'gerents' (I can't explain this - if you can you should give a lot more credence to this answer).
'after' says to put letters next to each other.
'one' becomes 'I' (Roman numeral).
'gerents' put after 'i' is 'igerents'.
'bell'+'igerents'='BELLIGERENTS'

Can you help me to learn more?