Shield unable to withstand blow? (7)

I believe the answer is:

buckler

'shield' is the definition.
('buckler' can be a synonym of 'shield')

I can't explain the remainder of the clue.

(Other definitions for buckler that I've seen before include "gives way under pressure?" , "Small shield for the forearm" , "Small round shield" , "likely to give way under pressure?" .)

I've seen this clue in The Independent.

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