The Great Gatsby's gun? (3)

I believe the answer is:

gat

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'gun?' is the definition.
(US informal term for a gun)

'the great gatsby's' is the wordplay.
'the great' becomes 'G' (abbreviation in place names).
'gatsby's' becomes 'at' (I can't explain this - if you can you should believe this answer much more).
'g'+'at'='GAT'

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(Other definitions for gat that I've seen before include "in Indian music, the last bit of raga" , "Piece" , "weapon" , "Revolver (informal)" , "Gangster's gun" .)

I've seen this clue in the Evening Standard.

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