Troublemakers are weak types, lacking weight (4)

I believe the answer is:

imps

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'troublemakers' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

(Other definitions for imps that I've seen before include "Mischievous characters" , "Little mischievous devils" , "Little devils or scamps" , "Mischievous spirits" , "Children causing trouble" .)

I've seen this clue in The Times.

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