Big beast is alone, sadly (3,4)
I believe the answer is:
sea lion
'big beast' is the definition.
(large aquatic animal)
'is alone sadly' is the wordplay.
'sadly' indicates anagramming the letters.
'is'+'alone'='isalone'
'isalone' anagrammed gives 'SEA-LION'.
(Other definitions for sea lion that I've seen before include "Large marine mammal" , "Large-eared marine mammal, sometimes performing" , "Large seal" , "Marine animal, used as a circus performer" , "Marine beast, the male having a mane" .)