Hallmark of the best amplifiers (5)

I believe the answer is:

stamp

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

'of the best amplifiers' is the wordplay.
'of' says the answer is hidden in the clue.
'STAMP' is hidden in the letters of 'best amplifiers'.

(Other definitions for stamp that I've seen before include "Philatelist's item" , "Tread heavily (on an envelope?)" , "Postage paid label" , "Hallmark" , "Imprint, brand" .)

I've seen this clue in the Evening Standard.

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