Animal sporting one horn? (5)
I believe the answer is:
rhino
'animal' is the definition.
(short for rhinoceros)
'sporting one horn?' is the wordplay.
'sporting' is an anagram indicator.
'one' becomes 'i' (Roman numeral).
'i'+'horn'='ihorn'
'ihorn' with letters rearranged gives 'RHINO'.
(Other definitions for rhino that I've seen before include "Animal hunted for its horn" , "Horned beast (abbr.)" , "Powerful herbivorous odd-toed ungulate, informally" , "Cash" , "Large animal with a horn on its snout, in short" .)