Serious as bodily harm can be, initially (8)

I believe the answer is:

grievous

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'serious' is the definition.
(I know that serious can be written as grievous)

I don't understand how the rest of the clue works.

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(Other definitions for grievous that I've seen before include "Very severe or painful" , "Severe, serious" , "The "G" of GBH" , "Legal assault charge" , "Calamitous" .)

I've seen this clue in the Evening Standard.

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