Bully not retreating in, ultimately, fist fight (6)
I believe the answer is:
tyrant
'bully' is the definition.
(I've seen this before)
'not retreating in ultimately fist fight' is the wordplay.
'not' becomes 'nary' (I've seen this before).
'retreating' shows that the letters should be reversed in order.
'in' indicates putting letters inside.
'ultimately' indicates one should take the final letters.
The final letters of 'fist fight' are 'tt'.
'nary' reversed gives 'yran'.
'yran' put within 'tt' is 'TYRANT'.
(Other definitions for tyrant that I've seen before include "Totalitarian" , "Despotic ruler" , "Autocrat or despot" , "Oppressor" , "Arbitrary ruler" .)