Exceptionally large safety measure (6)

I believe the answer is:

bumper

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'exceptionally large' is the definition.
(I've seen this before)

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

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(Other definitions for bumper that I've seen before include "Protection for car" , "Very large - vehicle protector" , "Glass filled to brim" , "Protective vehicle bar" , "First casualty in car crash" .)

I've seen this clue in The Mirror.

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