Toothed whale beheaded Spanish poet (4)
I believe the answer is:
orca
'toothed whale' is the definition.
(orca is a kind of toothed whale)
'beheaded spanish poet' is the wordplay.
'beheaded' means to remove the first letter.
'spanish poet' becomes 'lorca' (I've seen this before).
'lorca' with its initial letter taken away is 'ORCA'.
(Other definitions for orca that I've seen before include "Killer-whale" , "Killer whales" , "Massive killer" , "Sea creature" , "School killer" .)