Throw back tailless crustacean (7)
I believe the answer is:
lobster
'crustacean' is the definition.
(I know that lobster is a type of decapod)
'throw back tailless' is the wordplay.
'throw' becomes 'lob' (lobbing is a kind of throwing).
'back' becomes 'stern' (stern is a kind of back).
'tailless' means to remove the last letter (the tail of the word is removed).
'stern' with its last letter removed is 'ster'.
'lob'+'ster'='LOBSTER'
(Other definitions for lobster that I've seen before include "It turns bright-red when cooked" , "Crustacean caught in pot" , "could find this on the table" , "Large, strong-jawed crustacean" , "potted" .)