Killer a tailless bird upended (4)
I believe the answer is:
orca
'killer' is the definition.
(orca is a term for the killer whale)
'a tailless bird upended' is the wordplay.
'tailless' means to remove the last letter (the tail of the word is removed).
'bird' becomes 'crow' (crow is a kind of bird).
'upended' says the letters should be written in reverse (in a down clue, letters go up).
'crow' with its last letter removed is 'cro'.
'a'+'cro'='acro'
'acro' reversed gives 'ORCA'.
(Other definitions for orca that I've seen before include "Predatory black and white toothed whale" , "Large predator" , "27 [WHALE]" , "Killer-whale" , "ship" .)