Crossword Genius

This type of clue is back — without a couple of letters, it's a killer! (4)

Ross

I believe the answer is:

orca

'it's a killer' is the definition.
(orca is a term for the killer whale)

I cannot understand how the rest of the clue works.

(Other definitions for orca that I've seen before include "27 [WHALE]" , "ship" , "School killer" , "Large mammal" , "swimmer in black and white" .)

I've seen this clue in The Guardian.
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