Leo, 51, gets no return (4)
I believe the answer is:
lion
'leo' is the definition.
(I've seen this before)
'51 gets no return' is the wordplay.
'51' becomes 'LI' (roman numeral).
'gets' is a charade indicator (letters next to each other).
'return' is a reversal indicator.
'no' in reverse letter order is 'on'.
'li'+'on'='LION'
(Other definitions for lion that I've seen before include "Cat, the king of the beasts" , "King of the forest" , "Cowardly visitor in Oz" , "King of the jungle" , "Courageous person" .)