A creature so alien in form (3,4)
I believe the answer is:
sea lion
'a creature' is the definition.
(I have seen 'Marine animal' mean 'sea lion' so perhaps 'animal' could also mean 'sea lion')
'so alien in form' is the wordplay.
'in form' indicates anagramming the letters (I've seen this in other clues).
'so'+'alien'='soalien'
'soalien' with letters rearranged gives 'SEA-LION'.
(Other definitions for sea lion that I've seen before include "Aquatic mammal - code name for planned German invasion of England, 1940" , "Large marine mammal" , "Marine beast, the male having a mane" , "one often immersed in salty water" , "Creature" .)