Horned animal, one breaking crooked horn (5)
I believe the answer is:
rhino
'horned animal' is the definition.
(I've seen this before)
'one breaking crooked horn' is the wordplay.
'one' becomes 'i' (Roman numeral).
'breaking' indicates putting letters inside.
'crooked' indicates an anagram.
'horn' with letters rearranged gives 'rhno'.
'i' inserted into 'rhno' is 'RHINO'.
(Other definitions for rhino that I've seen before include "greens" , "Hi, Ron! (anag)" , "One charging" , "Horned animal; money (slang)" , "Charging animal" .)