Dignified knocking beer back (5)

I believe the answer is:

regal

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'dignified knocking' is the definition.
'regal' can be an answer for 'dignified' (thesaurus). I am not certain of the 'knocking' bit.

'beer back' is the wordplay.
'beer' becomes 'lager' (lager is a kind of beer).
'back' says the letters should be written backwards.
'lager' in reverse letter order is 'REGAL'.

Can you help me to learn more?

(Other definitions for regal that I've seen before include "Like a queen?" , "fit for the Queen" , "Portable organ" , "Kingly glare?" , "King-like" .)

I've seen this clue in The Telegraph.

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