Courteous but deranged poet clutching lb1 (6)

I believe the answer is:

polite

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'courteous' is the definition.
(similar in meaning)

'deranged poet clutching lb1' is the wordplay.
'deranged' is an anagram indicator.
'clutching' is an insertion indicator.
'lbl' becomes 'li' (I can't explain this - if you can you should believe this answer much more).
'poet' anagrammed gives 'pote'.
'pote' enclosing 'li' is 'POLITE'.

'but' acts as a link.

Can you help me to learn more?

(Other definitions for polite that I've seen before include "Mannerly, courteous" , "Considerate, cultured" , "Cultured, refined" , "Well-mannered, courteous" , "Of courteous manners" .)

I've seen this clue in the Evening Standard.

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