Like some idiots, slightly mad (5)

I believe the answer is:

dotty

'slightly mad' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for dotty that I've seen before include "like a Dalmatian?" , "Barking" , "Slightly crazy" , "Mildly eccentric" , "Slightly mad (colloq.)" .)

I've seen this clue in The Independent.

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