It's very bad when there's not a lean one left (5)

I believe the answer is:

fatal

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But I can't explain it at all! Could you help me understand?

(Other definitions for fatal that I've seen before include "Bringing death" , "Disastrous, maybe lethal" , "Calamitous" , "making one late" , "Deadly force" .)

I've seen this clue in the Evening Standard.

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