Greek character fencing in horned mammal (5)
I believe the answer is:
rhino
'horned mammal' is the definition.
(I've seen this before)
'greek character fencing in' is the wordplay.
'greek character' becomes 'rho' (letter from the Greek alphabet**).
'fencing' indicates putting letters inside (to fence can mean to contain or put a limit round).
'rho' placed around 'in' is 'RHINO'.
(Other definitions for rhino that I've seen before include "big game" , "one may charge unwary tourist?" , "wild beast" , "Hi, Ron, this animal is abbreviated" , "one might charge in Africa" .)