It's extremely handy for a sort of sketch (9)

I believe the answer is:

thumbnail

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'sort of sketch' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for thumbnail that I've seen before include "Small image" , "Particular sketch" , "Kind of sketch r" , "Brief description" , "small picture" .)

I've seen this clue in The Globe and Mail.

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