Having been assembling the Luger in the shade (4-5)

I believe the answer is:

blue-green

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'having been' is the definition.
I can't judge whether this definition defines the answer.

'assembling the luger in the shade' is the wordplay.
'assembling' indicates an anagram.
'in the' indicates putting letters inside.
'shade' becomes 'been' (I can't explain this - if you can you should give a lot more credence to this answer).
'luger' anagrammed gives 'luegr'.
'luegr' put inside 'been' is 'BLUE-GREEN'.

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I've seen this clue in the Evening Standard.

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