I sell drugs, but just exactly what is needed (5)

I believe the answer is:

ideal

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'just exactly what is needed' is the definition.
I can't judge whether this defines the answer.

'i sell drugs' is the wordplay.
'sell drugs' becomes 'deal' (I can't explain this - if you can you should believe this answer much more).
'i'+'deal'='IDEAL'

'but' is the link.

Can you help me to learn more?

(Other definitions for ideal that I've seen before include "Consummate" , "'Perfect, model (5)'" , "To which one aspires" , "without flaws" , "principle to be aimed at" .)

I've seen this clue in The Independent.

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