Crossword Genius

Losing key, killer becomes more cunning (5)

Ross

I believe the answer is:

slyer

I'm a little stuck... Click here to teach me more about this clue!

'more cunning' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for slyer that I've seen before include "More cunning" , "more sneaky" .)

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