Raced back with almost half a sea creature (7)
I believe the answer is:
narwhal
'a sea creature' is the definition.
(type of whale)
'raced back with almost half' is the wordplay.
'raced' becomes 'ran' ('run' can be a synonym of 'race').
'back' says the letters should be written in reverse.
'with' becomes 'w' (abbreviation).
'almost' means to remove the last letter (most of the word but not all of it).
'ran' written backwards gives 'nar'.
'half' with its last letter removed is 'hal'.
'nar'+'w'+'hal'='NARWHAL'
(Other definitions for narwhal that I've seen before include "Ocean mammal" , "Sea creature with a long spiral tusk" , "beast" , "Animal with large spiral tusk" , "swimmer" .)