Aquatic creature is alone sadly (3,4)
I believe the answer is:
sea lion
'aquatic creature' is the definition.
(sea animal)
'is alone sadly' is the wordplay.
'sadly' indicates an anagram.
'is'+'alone'='isalone'
'isalone' is an anagram of 'SEA-LION'.
(Other definitions for sea lion that I've seen before include "Is alone (anag.)" , "Large-eared mammal of Pacific Ocean, like Leo" , "Marine animal, used as a circus performer" , "Large seal" , "Large-eared marine mammal, sometimes performing" .)