Animal -- one encapsulated by extraordinary horn (5)
I believe the answer is:
rhino
'animal' is the definition.
(I've seen this before)
'one encapsulated by extraordinary horn' is the wordplay.
'one' becomes 'i' (Roman numeral).
'encapsulated by' indicates putting letters inside (I've seen this in other clues).
'extraordinary' indicates anagramming the letters.
'horn' anagrammed gives 'rhno'.
'i' inserted within 'rhno' is 'RHINO'.
(Other definitions for rhino that I've seen before include "endangered creature" , "chargeable > 11 [LOLLY]?" , "The necessary" , "Animal hunted for its horn" , "When provoked, one may charge" .)