Crossword Genius

Empty pledges added to Filbert's bad spirits (4)

Ross

I believe the answer is:

imps

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'bad spirits' is the definition.
'imps' can be an answer for 'spirits' (I have seen 'Mischievous spirits ' mean 'imps' so perhaps 'spirits' could also mean 'imps'). I am unsure of the 'bad' bit.

'empty pledges added to filbert's' is the wordplay.
'empty' means to remove the middle letters.
'added to' is a charade indicator (letters next to each other).
'filbert's' becomes 'I'm' (where the crossword setter is 'Filbert').
'pledges' with its middle taken out is 'ps'.
'ps' after 'im' is 'IMPS'.

Can you help me to learn more?

(Other definitions for imps that I've seen before include "Cheeky little devils" , "Mischievous spirits" , "Children causing trouble" , "Mischievous fairies" , "Mischievous sprites" .)

I've seen this clue in The Independent.
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