One going around with horn (5)
I believe the answer is:
rhino
'one going around with horn' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'one going around with horn' is the wordplay.
'one' becomes 'i' (Roman numeral).
'going around with' indicates anagramming the letters.
'i' mixed up with 'horn' can make 'RHINO'.
(Other definitions for rhino that I've seen before include "In brief, large thick-skinned animal of Africa" , "one may charge unwary tourist?" , "Animal hunted for its horn" , "Horned beast (abbr.)" , "Horny creature" .)