Brandish gun, a fine specimen (9)

I believe the answer is:

showpiece

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'a fine specimen' is the definition.

'brandish gun' is the wordplay.
'brandish' becomes 'show' (I can't explain this - if you can you should believe this answer much more).
'gun' becomes 'piece' (piece is a kind of gun).
'show'+'piece'='SHOWPIECE'

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(Other definitions for showpiece that I've seen before include "main feature" , "especially fine example" , "jewel" , "good example" , "shining example" .)

I've seen this clue in The Times.

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